Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane,south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting     

《Gondwana Research》2017

Post-collisional ultrapotassic magmatic rocks (15.2–18.8 Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo_87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo_77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5 wt%), Ni (145–346 ppm), Cr (289–610 ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (⁸⁷Sr/⁸⁶Sr)_i (0.7167–0.7274) and unradiogenic (¹⁴³Nd/¹⁴⁴Nd)_i (0.5118–0.5120), high (²⁰⁷Pb/²⁰⁴Pb)_i (15.740–15.816) and (²⁰⁸Pb/²⁰⁴Pb)_i (39.661–39.827) at a given (²⁰⁶Pb/²⁰⁴Pb)_i (18.363–18.790) with high δ¹⁸O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5 wt%) subgroup displays higher (¹⁴³Nd/¹⁴⁴Nd)_i, lower (⁸⁷Sr/⁸⁶Sr)_i and (²⁰⁶Pb/²⁰⁴Pb)_i ratios and lower δ¹⁸O values compared with the low-MgO (4.6–8.8 wt%) subgroup. High Ni (850–4862 ppm) contents of olivine phenocrysts and high whole-rock SiO₂, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO₂, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65 vol%) and orthopyroxene (30–50 vol%) with minor phlogopite (< 3 vol%) and quartz (< 2 vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75 vol%), orthopyroxene (20–35 vol%), clinopyroxene (< 3 vol%), phlogopite (< 2 vol%) and spinel (< 2 vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8 Ma in south Tibet.  相似文献   

Post-collisional potassic magmatism in the eastern Lhasa terrane,South Tibet: Products of partial melting of mélanges in a continental subduction channel     

《Gondwana Research》2017

Post-collisional, potassic magmatic rocks widely distributed in the eastern Lhasa terrane provide significant information for comprehensive understanding of geodynamic processes of northward subduction of the Indian lithosphere and uplift of the Tibetan Plateau. A combined dataset of whole-rock major and trace elements, Sr–Nd–Pb isotopes, and in situ zircon U–Pb dating and Hf–O isotopic analyses are presented for the Yangying potassic volcanic rocks (YPVR) in the eastern part of the Lhasa terrane, South Tibet. These volcanic rocks consist of trachytes, which are characterized by high K₂O (5.46–9.30 wt.%), SiO₂ (61.34–68.62 wt.%) and Al₂O₃ (15.06–17.36 wt.%), and relatively low MgO (0.47–2.80 wt.%) and FeO_t (1.70–4.90 wt.%). Chondrite-normalized rare earth elements (REE) patterns display clearly negative Eu anomalies. Primitive mantle-normalized incompatible trace elements diagrams exhibit strong enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and display significantly negative Nb–Ta–Ti anomalies. Initial isotopic compositions indicate relatively radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.711978–0.712090)] and unradiogenic Nd [(¹⁴³Nd/¹⁴⁴Nd)_i = 0.512121–0.512148]. Combined with their Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.615–18.774, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.708–15.793, (²⁰⁸Pb/²⁰⁴Pb)_i = 39.274–39.355)], these data are consistent with the involvement of component from subducted continental crustal sediment in their source region. The whole-rock Sr–Nd–Pb isotopic compositions exhibit linear trends between enriched mantle-derived mafic ultrapotassic magmas and relatively depleted crustal contaminants from the Lhasa terrane. The enrichment of the upper mantle below South Tibet is considered to result from the addition of components derived from subducted Indian continental crust to depleted MORB-source mantle during northward underthrusting of the Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ~ 55 Ma. Secondary Ion Mass Spectrometry (SIMS) U–Pb zircon analyses yield the eruptive ages of 10.61 ± 0.10 Ma and 10.70 ± 0.18 Ma (weighted mean ages). Zircon Hf isotope compositions [Ɛ_Hf(t) = −4.79 to −0.17], combined with zircon O isotope ratios (5.51–7.22‰), imply an addition of crustal material in their petrogenesis. Clinopyroxene-liquid thermobarometer reveals pressure (2.5–4.1 kbar) and temperature (1029.4–1082.9 °C) of clinopyroxene crystallization, suggesting that depth of the magma chamber was 11.6–16.4 km. Energy-constrained assimilation and fractional crystallization (EC–AFC) model calculation indicates depth of assimilation and fractional crystallization in the region of 14.40–18.75 km underneath the Lhasa terrane, which is in consistent with depth of the magma chamber as suggested by clinopyroxene-liquid thermobarometer. Based on the whole-rock major and trace elements and Sr–Nd–Pb isotope compositions, combined with EC–AFC modeling simulations and zircon Hf–O isotope data, we propose that the YPVR resulted from assimilation and fractional crystallization (AFC) process of the K-rich mafic primitive magmas, which were caused by partial melting of the Indian continental subduction-induced mélange rocks.  相似文献   

Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet     

《Gondwana Research》2017

New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ¹⁸O (6.7–11.3‰), low ε_Hf(t) (− 17.0 to − 12.0), and old Hf crustal model ages (1.87–2.19 Ga) that indicate an ancient SCLM source, and Group II rocks with δ¹⁸O values of 6.8–10.7‰, ε_Hf(t) values of − 11.8 to − 6.3, and younger Hf crustal model ages (1.50–1.86 Ga). The negative correlation defined by δ¹⁸O and ε_Hf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative ε_Hf(t) values between Group II samples (− 11.8 to − 6.3) and the High Himalayan gneiss (− 14.2 to + 0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.  相似文献   

Geochronology and geochemistry of Mesozoic mafic–ultramafic complexes in the southern Liaoning and southern Jilin provinces,NE China: Constraints on the spatial extent of destruction of the North China Craton     

Fu-Ping Pei  Wen-Liang Xu  De-Bin Yang  Yang Yu  Wei Wang  Quan-Guo Zhao 《Journal of Asian Earth Sciences》2011,40(2):636-650

LA–ICP–MS zircon U–Pb ages, geochemical and Sr–Nd–Pb isotope data are presented for mafic–ultramafic complexes from the southern Liaoning–southern Jilin area with the aim of determining the nature of the Mesozoic lithospheric mantle and to further constrain the spatial extent of destruction of the North China Craton (NCC). The complexes consist of olivine-websterite, gabbro, dolerite, and gabbro-diorite. Zircons from the complexes show typical zoning absorption, are euhedral–subhedral in shape, and yield high Th/U ratios (1.23–2.87), indicating a magmatic origin. Zircon U–Pb age data indicate that they formed in the Early Cretaceous (129–137 Ma). Geochemically, they have SiO₂ = 44.3–49.8%, MgO = 6.8–26.5%, Cr = 102–3578 ppm, and Ni = 31–1308 ppm, and are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), as well as a wide range of Sr–Nd–Pb isotopic compositions [(⁸⁷Sr/⁸⁶Sr)i = 0.70557–0.71119; ε_Nd (t) = ?5.4 to ?20.1; (²⁰⁶Pb/²⁰⁴Pb)i = 15.13–17.85; Δ7/4 = ?11.49 to 16.00; Δ8/4 = 102.64–203.48]. Compared with the southern Liaoning mafic–ultramafic rocks, the southern Jilin mafic–ultramafic rocks have high TiO₂ and Al₂O₃ contents, high ε_Nd (t) values, low (La/Yb)_N values, low initial ⁸⁷Sr/⁸⁶Sr ratios, and low radiogenic Pb isotopic compositions. These findings indicate that the primary magmas of the southern Jilin complexes were derived from lithospheric mantle that was previously metasomatized by a melt derived from the delaminated ancient lower crust, whereas the primary magmas of the southern Liaoning complexes originated from partial melting of a lithospheric mantle source that was previously modified by melt derived from the broken-off Yangtze slab. Therefore, the lateral extent of the NCC destruction should include the southern Liaoning–southern Jilin area.  相似文献   

Nd–Hf–Sr–Pb isotopes and trace element geochemistry of Proterozoic lamproites from southern India: Subducted komatiite in the source     

《Chemical Geology》2007,236(3-4):291-302

The probable sources of some of the famous Indian diamonds are the 1.2 Ga old Krishna lamproites of Southern India, a rare Proterozoic occurrence of lamproites which are usually Cretaceous or younger in age. In this study we report Nd, Sr, Pb and Hf isotopes and multiple trace element concentrations of the Krishna lamproites. The goals are to evaluate mantle-processes and the petrogenesis of these ultrapotassic rocks of extreme chemical composition in light of these geochemical data, including their major element compositions.The Krishna lamproites show nearly uniform, parallel rare earth element (REE) distribution patterns with high concentrations and extreme light-REE enrichment (La/Yb_(N) = 41–88), high average concentrations of Ba (∼ 1200 ppm), Sr (∼ 1200 ppm), Zr (∼ 930 ppm), La (∼ 230 ppm), high U/Pb and Th/U ratios with notable absence of any Eu-anomaly. These rocks are typically porphyritic without any evidence of crystal accumulation, and have moderately high Mg-numbers (59–73) along with high Ni (average ∼ 301 ppm, highest 819 ppm) and Cr (average ∼ 183 ppm, highest 515 ppm) concentrations that show a positive correlation with MgO (wt.%), implying a role of olivine in the melt source. The low SiO₂ content (lowest 37.8%, average 49%) and high Nb (average 147 ppm), Zr, Sr, as well as Ni and Cr in these rocks indicate lack of upper continental crustal contribution in the genesis of these rocks. The initial Pb-isotopic composition of these lamproites is unusual in that in a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb plot, these rocks plot to the left of the 1.2 Ga geochron (age of emplacement), unlike most mantle-derived rocks. This Pb-isotopic signature and the superchondritic Nb/Ta ratios (average 23.6) of these rocks rule out their derivation from a metasomatized sub-continental lithospheric mantle. The high ²⁰⁷Pb/²⁰⁴Pb at low ²⁰⁶Pb/²⁰⁴Pb indicates an Archean component in the source of these rocks. We argue that this Archean crustal component, which produced the low-SiO₂ lamproites along with the high Ni and Cr must have been ultrabasic, and we propose a model in which these lamproites formed by partial melting of metasomatized, subducted Archean komatiite in a peridotite mantle-source assemblage. In addition, these rocks display initial Hf isotopic compositions similar to Al-depleted komatiites, and high Nb/U, Nb/Th, and TiO₂ as well as low Al₂O₃/TiO₂ ratios (1.1–4.2) and average CaO/Al₂O₃ of ∼ 1.6 that are also similar to Archean komatiites. This is also supported by the initial Pb isotopic composition of the Krishna lamproites, requiring evolution in a variably high U/Pb, Th/Pb reservoir early in earth history, possibly resulting from preferential segregation of Pb relative to U and Th in the sulfides of the komatiite.The Al-depleted subducted komatiitic component was enriched by carbonate metasomatism in the peridotitic mantle. This metasomatism was responsible for the observed Nd–Hf isotope characteristics, specifically variable ε_Nd(T) at relatively constant ε_Hf(T) in the lamproites. This Nd–Hf-isotopic characteristic seems to be common in global lamproites of all ages. Our proposed model for the genesis of the Krishna lamproites involving a subducted komatiitic source may also be applicable for other global lamproites from cratonic settings, as older komatiite-bearing subducted crustal components were possibly ubiquitous in the architecture of ancient cratonic mantle.  相似文献   

Tectonic implications of Early Cretaceous low-Mg adakitic rocks generated by partial melting of thickened lower continental crust at the southern margin of the central North China Craton     

《Gondwana Research》2016

This paper reports new whole-rock geochemical, Sr–Nd–Pb isotopic, and zircon U–Pb and Hf isotopic data for Early Cretaceous intrusive rocks in the Sanmenxia–Houma area of central China, and uses these data to constrain the petrogenesis of low-Mg adakitic rocks (LMAR) and the spatial extent of the influence of the deeply subducted Yangtze slab during the Triassic evolution of this region. New zircon laser-ablation inductivity coupled plasma mass spectrometry (LA-ICP-MS) U–Pb data indicate that the early- and late-stage southern Quli, Qiligou, and Gaomiao porphyritic quartz diorites, the Canfang granodiorite, and the northern Wangmao porphyritic quartz monzodiorite were emplaced during the Early Cretaceous (~ 130 Ma) and the late Early Cretaceous (~ 116 Ma). These rocks are characterized by high Na₂O/K₂O, Sr/Y, and (La/Yb)_n ratios as well as high Sr concentrations, low Mg^# [molar 100 × Mg/(Mg + Fe^2 +_tot)] values, and low heavy rare earth element and Y concentrations, all of which indicate an LMAR affinity. The samples have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7054–0.7095), and low ε_Nd(t) (− 11.90 to − 22.20) and ε_Hf(t) (− 16.7 to − 32.7) values, indicative of a lower continental crust origin. The presence of Neoproterozoic (754–542 Ma) and inherited Late Triassic (220 Ma) metamorphic zircons within the late Early Cretaceous LMAR and the relatively high ²⁰⁶Pb/²⁰⁴Pb ratios of these rocks suggest that they formed from primary magmas derived from partial melting of Yangtze Craton (YC) basement material that had undergone ultrahigh-pressure metamorphism. In contrast, the presence of Paleoproterozoic and Archean inherited zircons within early Early Cretaceous LMAR in this area and the relatively low ²⁰⁶Pb/²⁰⁴Pb ratios of these rocks are indicative of derivation from primary magmas generated by partial melting of the thickened lower continental crust of the North China Craton (NCC). These rocks may have formed in an extensional environment associated with the upwelling of asthenospheric mantle material. The presence of YC basement material within the NCC in the Sanmenxia–Houma area suggests that the deeply subducted Yangtze slab influenced an area of ~ 100 km in lateral extent within the southern margin of the central NCC during the Triassic.  相似文献   

Evolution of K-rich magmas derived from a net veined lithospheric mantle in an ongoing extensional setting: Geochronology and geochemistry of Eocene and Miocene volcanic rocks from Eastern Pontides (Turkey)     

《Gondwana Research》2017

The Eocene and Miocene volcanic rocks between the cities of Trabzon and Giresun in the Eastern Pontides (NE Turkey) erupted as mildly and moderately alkaline magmas ranging from silica-saturated to silica-undersaturated types. ⁴⁰Ar-³⁹Ar dating and petrochemical data reveal that the studied volcanic rocks are discriminated in two: Lutetian (Middle Eocene) mildly alkaline, (basaltic rocks: 45.31 ± 0.18 to 43.86 ± 0.19 Ma; trachytic rocks: 44.87 ± 0.22 to 41.32 ± 0.12 Ma), and Messinian (Late Miocene) moderately alkaline volcanic rocks (tephrytic rocks: 6.05 ± 0.06 and 5.65 ± 0.06 Ma). The trace and the rare earth element systematic, characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios in the Eocene basaltic and trachytic rocks, high LREE/HREE ratios in the Miocene tephrytic rocks, and different degrees of depletion in Nb, Ta, Ti coupled with high Th/Yb ratios, show that the parental magmas of the volcanic rocks were derived from mantle sources previously enriched by slab-derived fluids and subducted sediments. The Sr, Nd and Pb isotopic composition of the Eocene and Miocene volcanic rocks support the presence of subduction-modified subcontinental lithospheric mantle. During the magma ascent in the crust, parental magmas of both the Eocene and Miocene volcanic rocks were mostly affected by fractional crystallisation rather than assimilation coupled with fractional crystallisation and mixing. The silica-undersaturated character of the Miocene tephrytic rocks could be attributed to assimilation of carbonate rocks within shallow-level magma chambers. The parental magmas of the Eocene volcanic rocks resulted from a relatively high melting degree of a net veined mantle and surrounding peridotites in the spinel stability field due to an increase in temperature, resulting from asthenospheric upwelling related to the extension of lithosphere subsequent to delamination. The parental magmas for the Miocene volcanic rocks resulted from a relatively low melting degree of a net veined mantle domain previously modified by metasomatic melts derived from a garnet peridotite source after decompression due to extensional tectonics, combined with strike-slip movement at a regional scale related to ongoing delamination.  相似文献   

Late Triassic alkaline complex in the Sulu UHP terrane: Implications for post-collisional magmatism and subsequent fractional crystallization     

《Gondwana Research》2016

Continental subduction and its interaction with overlying mantle wedge are recognized as fundamental solid earth processes, yet the dynamics of this system remains ambiguous. In order to get an insight into crust–mantle interaction triggered by partial melting of subudcted continental crust during its exhumation, we carried out a combined study of the Shidao alkaline complex from the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic (K₂O: 3.4–9.3 wt.%) gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field studies suggest that the mafic rocks are earlier than the felsic ones in sequence. LA-ICPMS zircon U–Pb dating on them gives Late Triassic ages of 214 ± 2 to 200 ± 3 Ma from mafic to felsic rocks. These ages are slightly younger than the Late Triassic ages (225–210 Ma) of the felsic melts from partial melting of the Sulu UHP terrane during exhumation. The alkaline rocks have wide ranges of SiO₂ (49.7–76.7 wt.%), MgO (8.25–0.03 wt.%), Ni (126.0–0.07 ppm), and Cr (182.0–0.45 ppm) contents. The contents of MgO, total Fe₂O₃, CaO, TiO₂ and P₂O₅ decrease with increasing SiO₂ contents. The contents of Na₂O, K₂O, and Al₂O₃ increase from gabbro to amphibole syenite, and decrease from amphibole syenite to granite, respectively. The alkaline rocks have characteristics of an arc-like pattern in trace element distribution, e.g., enrichment of LREE, LILE (Rb and Ba), Th and U, depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic rocks to the felsic rocks, the (La/Yb)_N ratios and the contents of the total REE, Sr and Ba decrease but the Rb contents increase. The alkaline rocks with high SiO₂ contents also display features of an A2-type granitoids, e.g., high contents of total alkalis, Zr and Nb and high ratios of Fe₂O₃^T/MgO, Ga/Al, Yb/Ta and Y/Nb, suggesting a post-collisional magmatism during exhumation of the Sulu UHP terrane. The alkaline rocks have homogeneous initial ⁸⁷Sr/⁸⁶Sr ratios (0.7058–0.7093) and negative ε_Nd(t) values (− 18.6 to − 15.0) for whole-rock. The Sr–Nd isotopic data remain almost unchanged with varying SiO₂ and MgO contents, suggesting a fractional crystallization (FC) process from the same parental magma. Our studies suggest a crust–mantle interaction in continental subduction interface as follows: (1) hydrous felsic melts from partial melting of subducted continental crust during its exhumation metasomatized the overlying mantle wedge to form a K-rich and amphibole-bearing mantle; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex under a post-collisional setting; and (3) the alkaline magma experienced subsequent fractionational crystallization mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar.  相似文献   

Petrogenesis of synorogenic high-temperature leucogranites (Damara orogen,Namibia): Constraints from U–Pb monazite ages and Nd,Sr and Pb isotopes     

《Gondwana Research》2014,26(4):1614-1626

Two suites of leucogranites were emplaced at 508 ± 5.9 Ma in the Okombahe District of the Damara belt (Namibia) synchronous with the peak of regional high-temperature metamorphism. The Sr (⁸⁷Sr/⁸⁶Sr_init: 0.707 to 0.711), Nd (εNd_init: − 4.5 to − 6.6), and Pb isotopic (²⁰⁶Pb/²⁰⁴Pb: 18.51–19.13; ²⁰⁷Pb/²⁰⁴Pb: 15.63–15.69; ²⁰⁸Pb/²⁰⁴Pb: 38.08–38.66) compositions indicate that these peraluminous S-type granites were derived from mid- to lower-crustal rocks, which are slightly different to the metapelitic rocks into which they intruded. Since the leucogranites are unfractionated and show no evidence for assimilation or contamination, they constrain the temperature and pressure conditions of their formation. Calculated Zr and LREE saturation temperatures of ca. 850 °C indicate high-temperature crustal melts. High Rb/Sr and low Sr/Ba ratios are consistent with biotite dehydration melting of pelitic source rocks. Qz–Ab–Or systematics reveal that melting and segregation for the least fractionated samples occurred at ca. 7 kbar corresponding to a mid-crustal level of ca. 26 km. However, there is no evidence for a mantle component that could have served as a local heat source for crustal melting. Therefore, the hot felsic magmas that formed close to the time of peak metamorphism are the result of long-lasting high temperature regional metamorphic conditions and intra-crustal collision.  相似文献   

Post-collisional carbonatite-hosted rare earth element mineralization in the Hongcheon area,central Gyeonggi massif,Korea: Ion microprobe monazite U‐Th‐Pb geochronology and Nd‐Sr isotope geochemistry     

《Ore Geology Reviews》2016

The Hongcheon area in the central Gyeonggi massif is a unique carbonatite locality in South Korea. The age and petrogenesis of this uncommon rock type and associated rare earth element (REE) mineralization still remain uncertain. The NNE trending, 20–50 m wide and ~ 2 km long Fe-REE ore bodies are hosted within a swarm of carbonatite dykes intruding Precambrian basement gneisses. The intrusive nature of the dykes, fenite alteration halos, exsolution intergrowths of constituent minerals and stable isotope data in the literature collectively attest to the ore formation by crystallization of carbonatite magma. The carbonatites are composed primarily of dolomite, ankerite, siderite, magnetite, monazite, apatite, strontianite and pyrite with subordinate quartz, barite, columbite, fergusonite and calcite. The principal carrier phase of REEs is monazite. The REE contents of monazite vary narrowly (TREO = 66.1–69.4 wt.%) irrespective of the textural occurrence. Although the monazite shows a sample-to-sample variation in La/Nd ratio, the textural varieties from each rock sample are similar with respect to this ratio. Thorium contents in monazite are consistently low (average = ca. 2500 ppm) with unusually high (average = ca. 2200) Th/U ratios. Sensitive high-resolution ion microprobe (SHRIMP) dating of monazite yielded a weighted mean ²⁰⁸Pb/²³²Th age of 232.9 ± 1.6 Ma, which agrees with a weighted mean ²⁰⁶Pb/²³⁸U age of 227.2 ± 8.3 Ma within uncertainties. This age, coupled with comparable intrusion ages documented for kimberlites and monzonite-syenite-gabbro-mangerite suite from central Korea, demonstrates the occurrence of mantle-derived alkaline igneous activities and associated REE mineralization following the North and South China collision. The intrusion of the Hongcheon carbonatite and potassic or ultrapotassic suite in central Korea may have resulted from the post-collisional detachment of the subducted slab and consequent upwelling of hot asthenosphere and melting of the overriding lithospheric mantle. Initial Nd‐Sr isotopic ranges of the Hongcheon carbonatite (ε_Nd = ca. − 26, ⁸⁷Sr/⁸⁶Sr = 0.703–0.706) and previous trace element data deny a petrogenetic linkage with the coeval silicate magmas. The metasomatism in the lithospheric mantle source of the Hongcheon carbonatite must have occurred in the distant past (> 1.7 Ga) to generate significantly negative ε_Nd values.  相似文献   

Multi-stage granitic magmatism during exhumation of subducted continental lithosphere: Evidence from the Wulong pluton,South Qinling     

《Gondwana Research》2014,25(3-4):1108-1126

Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30 Ma is recorded from the 225–235 Ma quartz diorite on the pluton margin, the ca. 218 Ma granodiorite in the intermediate zone, and the ca. 207 Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with ⁸⁷Sr/⁸⁶Sr(i) of 0.7044–0.7062, unradiogenic Nd (ε_Nd(t) values of − 6.1 to − 3.0, Nd model ages of 1.1–1.3 Ga, and moderately radiogenic Pb compositions (²⁰⁶Pb/²⁰⁴Pb(i) = 17.500–17.872, ²⁰⁷Pb/²⁰⁴Pb(i) = 15.513–15.549, ²⁰⁸Pb/²⁰⁴Pb(i) = 37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with ε_Hf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600 Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973 ppm), high Sr/Y ratios (mostly > 60) and negligible Eu anomalies (Eu/Eu* = 0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17 ppm), HREE (Yb = 0.95–1.7 ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet + amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO₂ (60.65–63.98 wt.%) and high TiO₂ (0.39–0.86 wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220 Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO₂ (69.16–70.82 wt.%), high Al₂O₃ (15.33–16.22 wt.%) and A/CNK values (mostly > 1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205 Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO₂ (67.68–70.29 wt.%), low TiO₂ (mostly < 0.35 wt.%) and high Sr/Y ratios of 86–151.  相似文献   

A reappraisal of the evolution of the palaeo-Pacific margin of Gondwana from the Pb and Os isotope systematics of igneous rocks from the southern Adelaide fold belt,South Australia     

《Gondwana Research》2017

We have undertaken Pb and Os isotopic analyses of igneous rocks from the southern Adelaide fold belt to further understanding of the Neoproterozoic – early Palaeozoic evolution of the palaeo-Pacific margin of Gondwana. The new Pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at Black Hill and Cambrai may have intruded much earlier than previously thought. Intrusion may even have occurred prior to deposition of the Normanville and Kanmantoo Group sediments and the onset of what is traditionally regarded as the Delamerian Orogeny. We tentatively term this earlier event the Hallett Cove orogeny. However, equivalent ages are common in the formerly adjacent Ross Orogen. When the Pb isotope signatures of the suites of gabbros, and I-, S-, and A-type granites are compared at 487 Ma they all span a very large range (²⁰⁶Pb/²⁰⁴Pb_i = 15.4–18.7, ²⁰⁷Pb/²⁰⁴Pb_i = 15.4–15.7, ²⁰⁸Pb/²⁰⁴Pb_i = 33.0–38.8) compared with other formerly adjacent terranes or the subsequent Lachlan fold belt. Similarly, ¹⁸⁷Os/¹⁸⁸Os_i ratios span 0.122–1.17 but include one gabbro and one rhyolite that have subchondritic ¹⁸⁷Os/¹⁸⁸Os_i ratios. When taken together, the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle. Rather, it appears that magmas parental to the gabbros and A-type rocks were derived from subduction-modified lithospheric mantle that has a median metasomatic age of 1.4 Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt.  相似文献   

Geology,geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc,SW China     

《Ore Geology Reviews》2016

The Hongshan Cu-polymetallic deposit is located in the southern Yidun arc in southwestern China, where both subduction-related (Late Triassic) and post-collisional (Late Cretaceous) porphyry–skarn–epithermal mineralization systems have been previously recognized. In this study, two distinct magmatic events, represented by diorite porphyry and quartz monzonite porphyry, have been revealed in the Hongshan deposit, with zircon SHRIMP U–Pb ages of 214 ± 2 Ma and 73.4 ± 0.7 Ma, respectively. The 73 Ma age is comparable to the Re–Os ages of 77 to 80 Ma of ore minerals from the Hongshan deposit, indicating that the mineralization is related to the Late Cretaceous quartz monzonite porphyries rather than Late Triassic diorite porphyries. The Late Triassic diorite porphyries belong to the high-K calc-alkaline series and show arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U and depletion in HFSEs, indicating that they were formed during the westward subduction of the Garzê–Litang Ocean. In contrast, the Late Cretaceous quartz monzonite porphyries show shoshonitic I-type geochemical characteristics, with high SiO₂, K₂O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr–Nd–Pb isotopic compositions (average (⁸⁷Sr/⁸⁶Sr)_i = 0.7085; ε_Nd(t) = − 6.0; ²⁰⁶Pb/²⁰⁴Pb = 19.064, ²⁰⁷Pb/²⁰⁴Pb = 15.738, ²⁰⁸Pb/²⁰⁴Pb = 39.733) suggest that the quartz monzonite porphyries originated from the partial melting of the ancient lower crust in response to underplating of mafic magma from subduction metasomatized mantle lithosphere, possibly triggered by regional extension in the post-collisional tectonic stage. The S isotopic compositions (δ³⁴S_V-CDT = 3.81‰ to 5.80‰) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.014 to 18.809, ²⁰⁷Pb/²⁰⁴Pb = 15.550 to 15.785, and ²⁰⁸Pb/²⁰⁴Pb = 38.057 to 39.468) of ore sulfides indicate that the sulfur and metals were derived from mixed mantle and crustal sources. It is proposed that although the Late Triassic magmatic event is not directly related to mineralization, it contributed to the Late Cretaceous mineralization system through the storage of large amounts of sulfur and metals as well as water in the cumulate zone in the mantle lithosphere through subduction metasomatism. Re-melting of the mantle lithosphere including the hydrous cumulate zone and ancient lower crust during the post-collisional stage produced fertile magmas, which ascended to shallow depths to form quartz monzonite porphyries. Hydrothermal fluids released from the intrusions resulted in porphyry-type Mo–Cu ores in and near the intrusions, skarn-type Cu–Mo ores in the country rocks above the intrusions, and hydrothermal Pb–Zn ores in the periphery.  相似文献   

Geochemistry and geochronology of porphyries from the Beiya gold–polymetallic orefield,western Yunnan,China     

《Ore Geology Reviews》2015

The Beiya gold–polymetallic orefield, with gold reserves of 305 t, is one of the most representative porphyry-skarn orefields in the Jinshajiang–Ailaoshan Cu–Au ore belt within the Sanjiang region of southwest China. The orefield contains seven deposits: the Wandongshan, Hongnitang, Dashadi, Bijiashan, Weiganpo, Matouwan, and Bailiancun deposits. In this paper we report on the geochemistry and geochronology of porphyries associated with mineralization from the seven deposits. The results show that all the porphyries have similar geochemistry, with high alkalinity, high contents of SiO₂, Al₂O₃, K₂O, and Sr, high K₂O/Na₂O ratios, low MgO, Y, and Yb contents, enrichments in Ba, K, and Pb, depletions in P, Ti, Nb, and Ta, and non-evident to weak Eu depletions (δEu = 0.42–0.99). In the SiO₂ vs. Th/Ce diagram, the porphyry samples are distributed in the area of thickened lower crust, and in the Sr/Y vs. Y and La/Yb vs. Yb diagrams, the porphyries broadly followed the batch-melting trend of amphibolite containing up to 10% garnet. LA-MC-ICP-MS zircon U–Pb dating analysis suggests that the porphyries were emplaced between 34.62 ± 0.25 and 36.72 ± 0.25 Ma. They were coeval with lamprophyres (34 to 36 Ma) in the Beiya area and with potassic–ultrapotassic intrusive rocks (40 to 35 Ma) within the Jinshajiang–Ailaoshan magmatic belt, indicating possible genetic relation between these rock types. We suggest that the porphyries in the Beiya gold–polymetallic orefield were derived from the partial melting of a K-rich mafic source in the thickened lower crust, with the melting triggered by asthenospheric upwelling following the removal of lower lithospheric mantle.  相似文献   

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications     

《Gondwana Research》2016

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.  相似文献   

Constraints of C–O–S–Pb isotope compositions and Rb–Sr isotopic age on the origin of the Tianqiao carbonate-hosted Pb–Zn deposit,SW China     

《Ore Geology Reviews》2013

The Tianqiao Pb–Zn deposit in the western Yangtze Block, southwest China, is part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province. Ore bodies are hosted in Devonian and Carboniferous carbonate rocks, structurally controlled by a thrust fault and anticline, and carried about 0.38 million tons Pb and Zn metals grading > 15% Pb + Zn. Both massive and disseminated Pb–Zn ores occur either as veinlets or disseminations in dolomitic rocks. They are composed of ore minerals, pyrite, sphalerite and galena, and gangue minerals, calcite and dolomite. δ³⁴S values of sulfide minerals range from + 8.4 to + 14.4‰ and display a decreasing trend from pyrite, sphalerite to galena (δ³⁴S_pyrite > δ³⁴S_sphalerite > δ³⁴S_galena). We interpret that reduced sulfur derived from sedimentary sulfate (gypsum and barite) of the host Devonian to Carboniferous carbonate rocks by thermal–chemical sulfate reduction (TSR). δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite range from –5.3 to –3.4‰ and + 14.9 to + 19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid was a mixture origin of mantle, marine carbonate rocks and sedimentary organic matter. Sulfide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.378 to 18.601, ²⁰⁷Pb/²⁰⁴Pb = 15.519 to 15.811 and ²⁰⁸Pb/²⁰⁴Pb = 38.666 to 39.571) that are plotted in the upper crust Pb evolution curve and overlap with that of Devonian to Carboniferous carbonate rocks and Proterozoic basement rocks in the SYG province. Pb isotope compositions suggest derivation of Pb metal from mixed sources. Sulfide minerals have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7125 to 0.7167, higher than Sinian to Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than basement rocks. Again, Sr isotope compositions are supportive of a mixture origin of Sr. They have an Rb–Sr isotopic age of 191.9 ± 6.9Ma, possibly reflecting the timing of Pb–Zn mineralization. C–O–S–Pb–Sr isotope compositions of the Tianqiao Pb–Zn deposit indicate a mixed origin of ore-forming fluids, which have Pb–Sr isotope homogenized before the mineralization. The Permian flood basalts acted as an impermeable layer for the Pb–Zn mineralization hosted in the Devonian–Carboniferous carbonate rocks.  相似文献   

Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic rocks: Sr–Nd–Pb isotope data from Roman Province and Southern Tuscany     

S. Conticelli  M. D’Antonio  L. Pinarelli  L. Civetta 《Mineralogy and Petrology》2002,74(2-4):189-222

Summary The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. ²⁰⁶Pb/²⁰⁴Pb values range between 18.50 and 19.15, ²⁰⁷Pb/²⁰⁴Pb values range between 15.63 and 15.70, and ²⁰⁸Pb/²⁰⁴Pb values range between 38.35 and 39.20. The general ε_Sr vs. ε_Nd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of ε_Sr vs. ε_Nd is still observed, with ε_Sr from −2 to +180 and ε_Nd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at ε_Sr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to ε_Sr and ε_Nd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO₂-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks. Received February 16, 2000; revised version accepted September 6, 2001  相似文献   

The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe,Dabie Orogen,China     

《Journal of Asian Earth Sciences》2013

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717 ± 38 Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221 ± 2 Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ¹⁸O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded ε_Nd (t) = −12 to −10 and T_DM = 2.2–2.8 Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2 Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (²⁰⁶Pb/²⁰⁴Pb)_i = 15.34–16.50, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.23–15.32, and (²⁰⁸Pb/²⁰⁴Pb)_i = 35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.  相似文献   

Geochronology and geochemistry of Cretaceous Nanshanping alkaline rocks from the Zijinshan district in Fujian Province,South China: Implications for crust–mantle interaction and lithospheric extension     

《Journal of Asian Earth Sciences》2014

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for Nanshanping alkaline rocks from the Zijingshan district in southwestern Fujian Province (the Interior or Western Cathaysia Block) of South China. The Nanshanping alkaline rocks, which consist of porphyritic quartz monzonite, porphyritic syenite, and syenite, revealed a Late Cretaceous age of 100–93 Ma. All of the rocks show high SiO₂, K₂O + Na₂O, and LREE but low CaO, Fe₂O₃^T, MgO, and HFSE (Nb, Ta, P, and Ti) concentrations. These rocks also exhibit uniform initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7078 to 0.7087 and ε_Nd(t) values of −4.1 to −7.2, thus falling within the compositional field of Cretaceous basalts and mafic dikes occurring in the Cathaysia Block. Additionally, these rocks display initial Pb isotopic compositions with a ²⁰⁶Pb/²⁰⁴Pb_i ratio of 18.25 to 18.45, a ²⁰⁷Pb/²⁰⁴Pb_i ratio of 15.63 to 15.67, and a ²⁰⁸Pb/²⁰⁴Pb_i ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (ε_Hf(t) = −11.7 to −3.2), which are different from those of the basement rocks, we suggest that Nanshanping alkaline rocks were primarily derived from a subduction-related enriched mantle source. High Rb/Sr (0.29–0.65) and Zr/Hf (37.5–49.2) but relatively low Ba/Rb (4.4–8.1) ratios suggest that the parental magmas of these rocks were most likely formed via partial melting of a phlogopite-bearing mantle source with carbonate metasomatism. The relatively high SiO₂ (62.35–70.79 wt.%) and low Nb/Ta (10.0–15.3) ratios, positive correlation between SiO₂ and (⁸⁷Sr/⁸⁶Sr)_I, and negative correlation between SiO₂ and ε_Nd(t) of these rocks suggest that the crustal materials were also involved in formation of the Nanshanping alkaline rocks. Combined with geochemical and isotopic features, we infer magmatic processes similar to AFC (assimilation and fractional crystallization) involving early fractionation of clinopyroxene and olivine and subsequent fractionation of biotite-dominated assemblages coupled with a lesser amount of crustal contamination, thereby forming the Nanshanping alkaline rocks. The Nanshanping alkaline rocks appear to be associated with an extensional environment in the Cathaysia Block. This extensional regime could have resulted in the slab break-off and rollback of the subducting paleo-Pacific plate and the upwelling of the asthenospheric mantle, which induced partial melting of the enriched lithospheric mantle in the Cretaceous.  相似文献   

Permian basaltic rocks in the Tarim basin,NW China: Implications for plume–lithosphere interaction     

《Gondwana Research》2011,19(4):596-610

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO₂ (2.43%–4.59%, weight percent), high Fe₂O₃ + FeO (12.63%–17.83%) and P₂O₅ (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)⁸⁷Sr/⁸⁶Sr ratio (∼ 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial ⁸⁷Sr/⁸⁶Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t and (²⁰⁸Pb/²⁰⁴Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.  相似文献