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1.
The Hashitu molybdenum deposit is located in the southern part of the Great Hinggan Range,NE China.Molybdenum mineralization is hosted by and genetically associated with monzogranite and porphyritic syenogranite.Sr-Nd-Pb isotopes of the intrusions show that the porphyritic syenogranite has initial~(87)Sr/~(86)Sr ratios of 0.70418-0.70952,ε_(Nd)(t)values of 1.3 to 2.1(t=143Ma),~(206)Pb/~(204)Pb ratios of 19.191-19.573,~(207)Pb/~(204)Pb ratios of 15.551-15.572,and~(208)Pb/~(204)Pb ratios of38.826-39.143.The monzogranite has initial~(87)Sr/~(86)Sr ratios of 0.70293-0.71305,ε_(Nd)(t)values of 1.1 to2.0(t=147 Ma),~(206)Pb/~(204)Pb ratios of 19.507-20.075,~(207)Pb/~(204)Pb ratios of 15.564-15.596,and~(208)Pb/~(204)Pb ratios of 39.012-39.599.The calculated Nd model ages(T_(DM))for monzogranite and porphyritic syenogranite range from 866 to 1121 Ma and 795 to 1020 Ma,respectively.The granitic rocks in the Hashitu area have the same isotope range as granites in the southern parts of the Great Hinggan Range.The isotope composition indicates that these granites are derived from the partial melting of a juvenile lower crust originating from a depleted mantle with minor contamination by ancient continental crust.The integrating our results with published data and the Late Mesozoic regional tectonic setting of the region suggest that the granites in the Hashitu area formed in an intra-continent extensional setting,and they are related to the thinning of the thickened lithosphere and upwelling of the asthenosphere.  相似文献   

2.
Bima Formation volcanic rocks, which record the history of Neo-Tethyan subduction, are found within the central and eastern segments of the southern Lhasa subterrane, Tibetan Plateau. Zircon UPb dating, whole-rock major and trace element analysis, and Sr–Nd–Pb–Hf isotopic compositions of Bima Formation volcanic rocks from the central segment of the southern Lhasa subterrane were used to constrain the magmatic and tectonic evolution of the Lhasa terrane during the early Mesozoic. Zircon UPb dating of five samples yielded consistent ages of 184.3 ± 2.4 to 176.8 ± 3.5 Ma. The dominant volcanic rock types within the Bima Formation are basalts, basaltic andesites, andesites, and dacites, which are enriched in the large-ion lithophile elements (e.g., Rb, Sr, and Ba) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). (87Sr/86Sr)t ratios are low (0.702900–0.704146), εNd(t) and εHf(t) values are high and positive (+4.4 to +6.9 and + 9.6 to +15.7, respectively), and Pb isotope ratios are homogeneous (initial 206Pb/204Pb = 18.28–18.40; 207Pb/204Pb = 15.53–15.56; 208Pb/204Pb = 38.21–38.38). Combining the new data with those from a previous study of Bima Formation volcanic rocks from the eastern segment of the southern Lhasa subterrane indicates that the Bima Formation formed between the Middle Triassic and Early Jurassic. It suggests that more widespread early Mesozoic volcanic rocks in the southern margin of the Lhasa terrane. The basaltic rocks of the Bima Formation were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids, and subsequently experienced fractional crystallization without significant crustal contamination. The andesitic and dacitic rocks were formed by fractional crystallization of the basaltic magma. Our study indicates that the Bima Formation volcanic rocks were generated within a continental island arc setting related to northward subduction of the Neo-Tethyan oceanic slab during the early Mesozoic.  相似文献   

3.
The Qimantage area of Northwest China lies in the western part of the East Kunlun Orogenic Belt, and is dominated by late Permian to Late Triassic granitoids. Among these, the Middle Triassic granitoids are mainly distributed south of the North Kunlun Fault, and consist of two main granitic assemblages: the Kaimuqi assemblage in the east and the Mositu assemblage in the west. To better constrain the Indosinian tectonic evolution of this area, we present data on the geochronology, geochemistry, and petrology of ore-bearing granodiorites from the Kaimuqi area in eastern Qimantage. The granodiorite samples have porphyritic or fine-grained textures. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating yields emplacement ages of 238–242 Ma, interpreted here as the result of the Middle Triassic magmatism. The granodiorites are mostly of the high-K calc-alkaline series, and are enriched in light rare earth elements, depleted in heavy rare earth elements such as Nb, Ta, P, and Ti, and have weak negative Eu (Eu/Eu*) anomalies. The Kaimuqi granodiorites have lower SiO2 and Sr contents, and higher Na2O/K2O ratios than the Mositu granodiorites. They also show initial 87Sr/86Sr ratios of 0.712151–0.715436, εNd(t) values of ?7.4 to ?6.3, and two-stage Nd model ages of 1.53–1.61 Ga. Together with their radiogenic Pb isotopic ratios for 206Pb/204Pb(t) (18.271–18.622), 207Pb/204Pb(t) (15.637–15.651), and 208Pb/204Pb(t) (38.452–37.870), these data indicate both mantle and crustal contributions to the source of the granodiorites. Field investigations show that Middle Triassic granitoids in both the Mositu and Kaimuqi assemblages contain large numbers of mafic microgranular enclaves, which supports an interpretation of mantle and crustal magmatic mixing. Based on a comparison of these results with data from coeval granites in the Mositu assemblage, we propose that the Middle Triassic granitoids in the Qimantage area were produced at ca. 240 Ma, as a result of the end of subduction and the initiation of collision during the Variscan–Indosinian orogeny. Magma mixing may be interpreted as the result of slab breakoff in a subduction zone environment, which led to fluid metasomatism and induced partial melting of an enriched lithospheric mantle, resulting in the formation of voluminous granitic magma.  相似文献   

4.
TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean 206Pb/238U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (87Sr/86Sr)i ranging from 0.7083 to 0.7093, low ?Nd(t) values from ?14.6 to ? 19.2, 206Pb/204Pb = 16.65–17.18, 207Pb/204Pb = 15.33–15.54, and 208Pb/204Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.  相似文献   

5.
《International Geology Review》2012,54(12):1389-1400
Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (87Sr/86Sr) i ranging from 0.7082 to 0.7087, low ?Nd(t) values from??17.0 to??17.5, 206Pb/204Pb from 17.14 to 17.18, 207Pb/204Pb from 15.44 to 15.55, and 208Pb/204Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.  相似文献   

6.
The Yangchang granite‐hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA‐ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc‐alkaline with K2O/Na2O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large‐ion lithophile elements (LILEs) and depleted in high‐field‐strength elements (HFSEs) with negative Nb and Ta anomalies in primitive‐mantle‐normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar 206Pb/204Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the εNd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo‐Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
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, Fe2O3T, 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 εNd(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/204Pbi ratio of 18.25 to 18.45, a 207Pb/204Pbi ratio of 15.63 to 15.67, and a 208Pb/204Pbi ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (εHf(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 εNd(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   

8.
《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/86Srinit: 0.707 to 0.711), Nd (εNdinit: − 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   

9.
The limited geochronology and geochemistry data available for the Early Cretaceous igneous rocks of the southern Gangdese Belt, southern Tibet, has resulted in the proposal of conflicting geodynamic models for the generation of the widespread Cretaceous igneous rocks in the middle and northern parts of the belt. To explore this issue, we present SHRIMP U–Pb zircon data and geochemical and Sr–Nd–Pb–Hf isotopic data for the Mamen andesites from the southern margin of the Gangdese Belt. The Mamen andesites, emplaced at 136.5 Ma, are sodic (Na2O/K2O = 1.2–2.3) and have geochemical characteristics typical of adakites (i.e., high Al2O3, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial (87Sr/86Sr)t ratios of 0.70413–0.70513, positive εNd(t) values of 3.7–5.8, and (206Pb/204Pb)t ratios of 18.37–18.51, (207Pb/204Pb)t ratios of 15.59–15.65, and (208Pb/204Pb)t ratios of 38.43–38.72. In situ Hf isotopic analyses of zircons that had previously been dated by SHRIMP yielded positive initial εHf(t) values ranging from +11.0 to +15.5. A model calculation using trace element and Sr–Nd–Pb isotopic data indicates that several percent of subducted sediment is required to generate the Mamen andesites, which were derived via the partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridized by peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan oceanic crust was subducted northward beneath the Gangdese Belt during the Early Cretaceous at a high angle. Our results are inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust in generating the widespread Cretaceous magmatism recorded in the Gangdese Belt.  相似文献   

10.
The Taoxihu deposit (eastern Guangdong, SE China) is a newly discovered Sn polymetallic deposit. Zircon U-Pb dating yielded 141.8 ± 1.0 Ma for the Sn-bearing granite porphyry and 145.5 ± 1.6 Ma for the biotite granite batholith it intruded. The age of the granite porphyry is consistent (within error) with the molybdenite Re–Os isochron age (139.0 ± 1.1 Ma) of the Sn mineralization, indicating a temporal link between the two. Geochemical data show that the granite porphyry is weakly peraluminous, contain high Si, Na and K, low Fe, Mg, Ca and P, and relatively high Rb/Sr and low K/Rb values. The rocks are enriched in Rb, Th, U, K, and Pb and depleted in Ba, Sr, Ti and Eu, resembling highly fractionated I-type granites. They contain bulk rock initial 87Sr/87Sr of 0.707371–0.707730 and εNd(t) of −5.17 to −4.67, and zircon εHf(t) values from −6.67 to −2.32, with late Mesoproterozoic TDM2 ages for both Nd and Hf isotopes. This suggests that the granite porphyry was likely formed by the partial melting of the crustal basement of Mesoproterozoic overall residence age with minor mantle input.δ34SCDT values of the Taoxihu chalcopyrite and pyrite range from 0.1 to 2.1‰ (average: 0.9‰), implying a dominantly magmatic sulfur source. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the Taoxihu sulfide ores are 18.497–18.669, 15.642–15.673 and 38.764–38.934, respectively, indicating a mainly upper continental crustal lead source with minor mantle contribution. The highly fractionated and reduced (low calculated zircon Ce4+/Ce3+ and EuN/EuN1 values) nature of the ore-forming granitic magma may have facilitated the Sn enrichment and played a key role in the Sn mineralization. We propose that the ore-forming fluids at Taoxihu were of magmatic-hydrothermal origin derived from the granite porphyry, and that both the granite porphyry and the Sn mineralization were likely formed in an extensional setting, possibly related to the subduction slab rollback of the Paleo-Pacific Plate.  相似文献   

11.
Whole rock elemental and Sr–Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810–580 Ma), and is dominated by crust-derived relatively low-T (850–750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (−8 to −10) and highest mg# (30–40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96–1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595–580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (−12 to −18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0–2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714–0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd–Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites.  相似文献   

12.
The Jurassic magmatic and volcanic rocks are widespread along the west central Lhasa subterrane. However, the petrogenesis of these rocks is poorly understood because of lacking high-quality geochronology and geochemical data. Here, we present new zircon U–Pb age and Hf isotopic data, whole-rock geochemical and Sr–Nd–Pb isotopic data for the Songduole and Qiangnong plutons in Geji area. LA-ICP-MS dating of zircon yield crystallization ages of 172.1 ± 1.9 and 155.9 ± 1.2 Ma for the Songduole and Qiangnong plutons, respectively. Geochemically, Songduole and Qiangnong granodiorite are characterized by high MgO (2.63–3.49 wt%), high Mg# (49–50), and low TiO2 (0.48–0.57 wt%). Besides, all rocks show metaluminous, calc-alkaline signatures, with strong depletion of Nb, Ta, and Ti, enrichment of large-ion lithophile (e.g. Rb, Th, K), and a negative correlation between SiO2 and P2O5. All these features are indicative of arc-related I-type magmatism. Five samples from the Songduole granodiorite have whole rock (87Sr/86Sr)i of 0.71207–0.71257, εNd(t) values of ?15.1 to ?13.9, zircon εHf(t) values of ?17.4 to ?10.5, (206Pb/204Pb)t ratios of 18.402–18.854, (207Pb/204Pb)t ratios of 15.660–15.736, and (208Pb/204Pb)t ratios of 38.436–39.208. Samples from the Qiangnong granodiorite have (87Sr/86Sr)i of 0.71230–0.71252, εNd(t) values of ?15.1 to ?14.2, zircon εHf(t) values of ?12.6 to ?6.4, (206Pb/204Pb)t ratios of 18.688–18.766, (207Pb/204Pb)t ratios of 15.696–15.717, and (208Pb/204Pb)t ratios of 38.546–39.083. These geochemical signatures indicate that the two plutons most likely originated from partial melting of the ancient Lhasa lower crust with obvious inputs of mantle-derived melts. Combined with regional geology, our results indicate that the Jurassic magmatism in the west central Lhasa subterrane most likely resulted from the southward subduction of the Bangong Ocean lithosphere beneath the central Lhasa terrane.  相似文献   

13.
An isotopic study was systemically carried out on the granitic complex, diorite-porphyrite, ores and ore minerals of the 103 Ma Xiaoxinancha gold-rich copper deposit in Jilin province to determine the geodynamic model of diagenesis and metallogenesis. Results show that the initial Nd and Sr isotopic compositions of the granitic complex are in the range of 0.70425–0.70505 for (87Sr/86Sr)i , 0.51243–0.51264 for INd, and –1.31 to +2.64 for εNd(t); those of the diorite-porphyrite are in the range from 0.70438–0.70448 for (87Sr/86Sr)i, 0.51259–0.51261 for INd, and +1.56 to +2.09 for εNd(t). For ores and sulfides, the (87Sr/86Sr)i , INd, and εNd(t) values are in the range from 0.70440–0.70805, 0.51259–0.51279 and +1.72 to +5.56, respectively. The Pb isotopic ratios of the granitic complex range from 18.2992–18.6636 for 206Pb/204Pb, from 15.5343–15.5660 for 207Pb/204Pb, and from 38.1640–38.5657 for 208Pb/204Pb. For diorite-porphyrite, the isotopic ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 18.3919, 15.5794 and 38.3566, respectively, whereas those of the ores and ore sulfides vary from 18.2275–18.3770 for 206Pb/204Pb, from 15.5555–15.5934 for 207Pb/204Pb and from 38.1318–38.3131 for 208Pb/204Pb. The results indicate that the mineralization was correlated to the formation and evolution of the granitic complex and the diorite-porphyrite. Combining with the reported data in petrologic characteristics, elemental geochemistry and chronology, conclusions can be drawn that the geodynamic settings of diagenesis and metallogenesis of this deposit were consistent with the subduction of the Izanagi oceanic plate during the Early Cretaceous. The diorite-porphyrite was formed by the emplacement of the adakitic magma triggered by partial melting of the enriched mantle, which originated from the derivative continental lithospheric mantle metasomatized by dehydration fluids from the subducting oceanic crust. The granitic complex was produced by fractional crystallization of the mixture between the adakitic magma and the high-K calc-alkaline acidic magma, which were generated by the remelting of the lower crust in the course of intraplate upwelling of the adakitic magma. The ore-bearing fluid reservoir convened in a late stage of the evolution of the mixed magma chamber.  相似文献   

14.
U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).  相似文献   

15.
The Betulia Igneous Complex (BIC) is a group of Late-Miocene (11.8 ± 0.2 Ma) hypabyssal intrusions of intermediate to felsic composition located in the SW of the Colombian Andes. These bodies have a calc-alkaline tendency and are related to the subduction of the Nazca plate under the South American plate. Diorites, quartz diorites and tonalities have porphyritic and phaneritic textures and are composed of plagioclase, amphibole, quartz, biotite, and orthoclase. Plagioclase is mainly of andesine-type and the amphiboles were classified mainly as magnesiohornblendes, actinolites, and tschermakites.BIC rocks have a narrow range of SiO2 content (59–67wt%) and exhibit an enrichment of LILE and LREE relative to HFSE and HREE, respectively. These features are attributed to enrichment of LILE from the source and retention of HFSE (mainly Nb, Ta, and Ti) by refractory phases within the same source. The depletion of HREE is explained by fractionation of mineral phases that have a high partition coefficients for these elements, especially amphiboles, the major mafic phase in the rocks. Nevertheless, the fractionation of garnet in early stages of crystallization is not unlikely. Probably all BIC units were generated by the same magma chamber or at least by the same petrologic mechanism as shown by the similar patterns in spider and REE diagrams; fractional crystallization and differentiation processes controlled the final composition of the rocks, and crystallization stages determined the texture.Isotopic compositions of BIC rocks (87Sr/86Sr: 0.70435–0.70511; 143Nd/144Nd: 0.51258–0.51280; 206Pb/204Pb: 19.13–19.31; 207Pb/204Pb: 15.67–15.76; 208Pb/204Pb: 38.93–39.20) indicate a source derived from the mantle with crustal contamination. The model proposed for the BIC consists of fluids from the dehydration of the subducted slab (Nazca plate) and subducted sediments that generated partial melting of the mantle wedge. These basaltic melts ascended to the mantle–crust boundary where they were retained due to density differences and began to produce processes of melting, assimilation, storage, and homogenization (MASH zone). At this depth (∼40–45 km), fractional crystallization and differentiation processes began to produce more felsic magmas that were able to ascend through the crust and be emplaced at shallow depths.  相似文献   

16.
《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   

17.
Mineral chemistry, major and trace elements, 40Ar/39Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. 40Ar/39Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (LaN/LuN = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial 87Sr/86Sr values vary between .70615 and .70796, whereas initial 143Nd/144Nd values change between .51228 and .51249. Initial 206Pb/204Pb values vary between 18.001 and 18.349, 207Pb/204Pb values between 15.611 and 15.629 and 208Pb/204Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.  相似文献   

18.
Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia ( 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sri = 0.7048, Nd = –1.8,206Pb/204Pb(i) = 17.704 and207Pb/204Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites ( 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sri = 0.7100, Nd = –8.4,206Pb/204Pb(i) = 15.403 and207Pb/204Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage ( 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sri = 0.7114, Nd = –13.1,207Pb/204Pb(i) = 16.913 and207Pb/204Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.  相似文献   

19.
《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 εNd(T) at relatively constant εHf(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   

20.
The Wunugetushan porphyry Cu–Mo deposit is located in the Manzhouli district of NE China, on the southern margin of the Mesozoic Mongol–Okhotsk Orogenic Belt. Concentric rings of hydrothermal alteration and Cu–Mo mineralization surround an Early–Middle Jurassic monzogranitic porphyry. The Cu–Mo mineralization is clearly related to the quartz–potassic and quartz–sericite alteration. Molybdenite Re–Os and groundmass 40Ar/39Ar of the host porphyry dates indicate that the ore-formation and porphyry-emplacement occurred at 177.6 ± 4.5 Ma and 179.0 ± 1.9 Ma, respectively. Geochemically, the host porphyry of the deposit is characterized by strong LREE/HREE fractionation, enrichment in LILE, Ba, Rb, U, Th and Pb, and depletion of HFSE, Nb, Ta, Ti and HREE. The Sr–Nd–Pb isotopic compositions of the porphyry display an varied initial (87Sr/86Sr)i ratio, a positive εNd(t) values and high 206Pb/204Pbt, 207Pb/204Pbt and 208Pb/204Pbt ratios. These data indicate that the magmatic source of the host porphyry comprised two end-members: lithospheric mantle metasomatized by fluids derived from the subducted slab; and continental crust. We infer that the primitive magma of the host porphyry was derived from crust–mantle transition zone. Based on regional geology and geochemistry of the host porphyry, the Wunugetushan deposit is suggested to form in a continental collision environment after closure of the Mongol–Okhotsk Ocean.  相似文献   

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