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1.
The Central Asian Orogenic Belt (CAOB) formed mainly in the Paleozoic due to the closure of the Paleo-Asian oceanic basins and accompanying prolonged accretion of pelagic sediments, oceanic crust, magmatic arcs, and Precambrian terranes. The timing of subduction–accretion processes and closure of the Paleo-Asian Ocean has long been controversial and is addressed in a geochemical and isotopic investigation of mafic rocks, which can yield important insight into the geodynamics of subduction zone environments. The Xilingol Complex, located on the northern subduction–accretion zone of the CAOB, mainly comprises strongly deformed quartzo-feldspathic gneisses with intercalated lenticular or quasi-lamellar amphibolite bodies. An integrated study of the petrology, geochemistry, and geochronology of a suite of amphibolites from the complex constrains the nature of the mantle source and the tectono-metamorphic events in the belt. The protoliths of these amphibolites are gabbros and gabbroic diorites that intruded at ca. 340–321 Ma with positive εHf(t) values ranging from + 2.89 to + 12.98. Their TDM1 model ages range from 455 to 855 Ma and peak at 617 Ma, suggesting that these mafic rocks are derived from a depleted continental lithospheric mantle. The primitive magma was generated by variable degrees of partial melting of spinel-bearing peridotites. Fractionation of olivine, clinopyroxene and hornblende has played a dominant role during magma differentiation with little or no crustal contamination. The mafic rocks are derived from a Late Neoproterozoic depleted mantle source that was subsequently enriched by melts affected by slab-derived fluids and sediments, or melts with a sedimentary source rock. The Carboniferous mafic rocks in the northern accretionary zone of the CAOB record a regional extensional event after the Early Paleozoic subduction of the Paleo-Asian Ocean. Both addition of mantle-derived magmas and recycling of oceanic crust played key roles in significant Late Carboniferous (ca. 340–309 Ma) vertical crustal growth in the CAOB. Amphibolite–facies metamorphism (P = 0.34–0.52 GPa, T = 675–708 °C) affected these mafic rocks in the Xilingol Complex at ca. 306–296 Ma, which may be related to the crustal thickening by northward subduction of a forearc oceanic crust beneath the southern margin of the South Mongolian microcontinent. The final formation of the Solonker zone may have lasted until ca. 228 Ma. 相似文献
2.
The time of termination of orogenesis for the southern Altaids has been controversial. Systematic investigations of field geology, geochronology and geochemistry on newly discriminated mafic–ultramafic rocks from northern Alxa in the southern Altaids were conducted to address the termination problem. The mafic–ultramafic rocks are located in the Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km. All rocks occur high-grade gneisses as tectonic lenses that are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have undergone pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by uniform compositional trends, i.e., with low SiO2-contents (42.51–52.21 wt.%) and alkalinity (Na2O + K2O) (0.01–5.45 wt.%, mostly less than 0.8 wt.%), and enrichments in MgO (7.37–43.36 wt.%), with Mg# = 52.75–91.87. As the rocks have been strongly altered and have a wide range of loss-on-ignition (LOI: 0.44–14.07 wt.%) values, they may have been subjected to considerable alteration by either seawater or metamorphic fluids. The REE and trace element patterns show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc origin. The ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid-ocean-ridge magmatism. LA-ICP MS U–Pb zircon ages of gabbros from three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of the mafic–ultramafic rocks. Therefore, considering other previously published data, we suggest that the mafic–ultramafic rocks were products of south-dipping subduction, most probably with a slab window caused by ridge subduction, of the Paleo-Asian Ocean plate beneath the Alxa block in the Late Carboniferous to Late Permian before the Ocean completely closed. This sheds light on the controversial tectonic history of the southern Altaids and supports the concept that the termination of orogenesis was in the end-Permian to Triassic. 相似文献
3.
《Gondwana Research》2011,19(4):638-652
Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean 206Pb/238U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO2 contents (45.2–52.7 wt.%) and total alkaline (K2O + Na2O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/YbN = 0.86–1.1), together with their high εNd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/YbN = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low εNd(t) values (+ 3.4 to + 5.4) and high initial 87Sr/86Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give εHf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai. 相似文献
4.
《Precambrian Research》2007,152(1-2):27-47
Metasomatism above subduction zones is an important process that produces heterogeneous mantle and thus a diversity of igneous rocks. In the Panzhihua district, on the western margin of the Yangtze Block (SW China), two Neoproterozoic mafic intrusions, one olivine gabbro and one hornblende gabbro, have identical ages of 746 ± 10 and 738 ± 23 Ma. Both of the gabbros are tholeiitic in composition and have arc-like geochemical compositions. The hornblende gabbros have K2O concentrations ranging from 0.70 to 1.69 wt.% and show enrichment of Rb, Ba, U, Th and Pb and depletion of Nb,Ta and Ti. They have variable 87Sr/86Sr ratios (0.7045–0.7070) with constant ɛNd(t) values (−0.12 to −0.93). The olivine gabbros have relatively low K2O (0.19–0.43 wt.%), are depleted in Rb and Th relative to Ba and U, and have obvious negative Nb–Ta and Zr–Hf anomalies on primitive mantle-normalized trace element diagrams. Their ɛNd(t) values range from −0.64 to −1.73 and initial 87Sr/86Sr ratios from 0.7070 to 0.7075. Both types of gabbro experienced fractional crystallization of clinopyroxene, plagioclase, amphibole and minor Fe–Ti oxide. The parental magmas of the olivine and hornblende gabbros were formed by about 20% partial melting of garnet–spinel lherzolite and spinel lherzolite, respectively. According to trace elemental ratios, the hornblende gabbros were probably derived from a source strongly modified by subducted slab fluids, whereas the olivine gabbros came from a mantle source modified by subducted slab melts. The close association of the olivine gabbros and hornblende gabbros suggests that a steep subduction zone existed along the western margin of the Yangtze Block during Neoproterozoic time. Thus, the giant Neoproterozoic magmatic event in South China was subduction-related. 相似文献
5.
《Chemie der Erde / Geochemistry》2017,77(1):183-194
The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO2 (51.81–56.71 wt.%), low K2O (0.08–1.65 wt.%) and high Na2O (4.3–5.03 wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K2O, high Na2O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO2 concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1 wt% of TiO2 and the ternary plots of A (Na2O + K2O) F(FeOT) M(MgO) and TiO2-K2O-SiO2/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (LaN/SmN: 2.37–3.62) and flat HREE (Eu/Eu*: 0.90–1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50–100 times chondrite with slight enrichment of LREE (LaN/SmN = 2.21–3.13) and flat HREE (Eu/Eu*: 0.94–1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge. 相似文献
6.
Mafic volcanic rocks in the 3.8–3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called ‘ocelli’. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9–21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO2 = 62.9–72.0 wt.%; MgO = 0.60–3.50 wt.%; Ni = 58–143 ppm; Cr = 250–510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO2 = 46.6–50.6 wt.%; MgO = 8.70–12.30 wt.%; Ni = 119–175 ppm; Cr = 330–600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO2 = 54.2–60.7 wt.%; MgO = 3.95–7.72 wt.%; Ni = 127–158 ppm; Cr = 500–570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts.On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Smcn = 0.30–0.94; Gd/Ybcn = 1.03–1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Smcn = 1.30–2.60; Gd/Ybcn = 1.32–2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma–crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean. 相似文献
7.
《Chemie der Erde / Geochemistry》2017,77(3):517-533
The northern Noorabad area in western Iran contains several gabbro and basalt bodies which were emplaced along the Zagros suture zone. The basalts show pillow and flow structures with amygdaloidal textures, and the gabbroic rocks show massive and foliated structures with coarse to fine-grained textures. The SiO2 contents of the gabbros and basalts are similar and range from 46.1–51.0 wt.%, and the Al2O3 contents vary from 12.3–18.8 wt.%, with TiO2 contents of 0.4–3.0 wt.%. The Nb concentrations of some gabbros and basalts are high and can be classified as Nb-enriched arc basalts. The positive εNd(t) values (+3.7 to +9.8) and low 87Sr/86Sr(initial) ratios (0.7031–0.7071) of both bodies strongly indicate a depleted mantle source and indicate that the rocks were formed by partial melting of a depleted lithospheric mantle and interaction with slab fluids/melts. The chemical composition of trace elements, REE pattern and initial 87Sr/86Sr-143Nd/144Nd ratios show that the rocks have affinities to tholeiitic magmatic series and suggest an extensional tectonic regime over the subduction zone for the evolution of these rocks. We propose an extensional tectonic regime due to the upwelling of metasomatized mantle after the late Cretaceous collision in the Harsin-Noorabad area. These rocks can be also considered as Eocene back arc magmatic activity along the Zagros suture zone in this area. 相似文献
8.
The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Smcn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Ybcn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Smcn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes. 相似文献
9.
The Manipur Ophiolite Complex (MOC) located in the Indo-Myanmar Orogenic Belt (IMOB) of Northeast India forms a section of the Tethyan Ophiolite Belt of the Alpine–Himalayan orogenic system. Whole rock compositions and mineral chemistry of mantle peridotites from the MOC show an affinity to the abyssal peridotites, characterized by high contents of Al2O3 (1.28–3.30 anhydrous wt.%); low Cr# of Cr-spinel (0.11–0.27); low Mg# of olivine (∼Fo90) and high Al2O3 in pyroxenes (3.71–6.35 wt.%). They have very low REE concentrations (∑REE = 0.48–2.14 ppb). Lherzolites display LREE-depleted patterns (LaN/SmN = 0.14–0.45) with a flat to slightly fractionated HREE segments (SmN/YbN = 0.30–0.65) whereas Cpx-harburgites have flat to upward-inflected LREE patterns (LaN/SmN = 0.13–1.23) with more fractionated HREE patterns (SmN/YbN = 0.13–0.65) than the lherzolite samples. Their platinum group elements (PGE) contents (<50 ppb) and distinct mantle-normalised PGE patterns with the Pd/Ir values (1.8–11.9) and Pt/Pt* values (0.2–1.1) show an affinity to the characteristic of the residual mantle material. Evaluation of mineralogical and petrological characteristics of these peridotites suggests that they represent the residues remaining after low degree of partial melting (∼2–12%) in the spinel stability field of a mid-oceanic ridge environment. The well-preserved mid-oceanic ridge characteristics of these peridotites further suggest that the mantle section was subsequently trapped in the forearc region of the subduction zone without undergoing significant modification in their chemistry by later subduction-related tectonic and petrological processes before its emplacement to the present crustal level. 相似文献
10.
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 εNd(t) and εHf(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
11.
《Journal of South American Earth Sciences》2007,24(4):271-289
The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average LaN/YbN = 8.41) and significant negative Eu anomalies (average Eu/Eu1 = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average LaN/YbN = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu1 = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average LaN/YbN = 15.35), resembling those of Group 1, and an identical average Eu/Eu1 of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu1 = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and GdN/YbN–Eu/Eu1 diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K2O/Na2O, LaN/YbN, LaN/SmN, Eu/Eu1, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent. 相似文献
12.
《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 (εNd(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 εHf(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
13.
《Chemie der Erde / Geochemistry》2016,76(4):567-578
Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO2 (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO2, P2O5, MnO, Al2O3 and Fe2O3 define linear trends with negative slopes against SiO2, whilst Na2O and K2O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO2, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (LaN/YbN = 4.5–6.4) with moderate HREE fractionation (SmN/YbN = 1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (< 400 ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the 87Sr/86Sr initial ratios vary from 0.704432 to 0.705989, and the 143Nd/144Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks. 相似文献
14.
《地学前缘(英文版)》2023,14(3):101534
Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (～272 Ma), followed by picritic ultramafic rocks (～270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (εHf(t) = +13.1–+16.9; εNd(t) = +6.9–+7.1), transitional mafic rocks (εHf(t) = +1.8–+16.9; εNd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (εHf(t) = +0.5–+5.4; εNd(t) = ?1.5 to +1.9) and picritic ultramafic rocks (εHf(t) = +14.9–+17.2; εNd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume. 相似文献
15.
The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeOtot and Cr2O3 of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu1 = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive εNd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc. 相似文献
16.
The Xiaohuangshan ophiolite of the Beishan (Inner Mongolia) is located in the southern margin of the Central Asian Orogenic Belt. It consists of several blocks composed dominantly of serpentinized ultramafic rocks, cumulative gabbros and basalts. The geochemical data of gabbros and basalts obtained from the Xiaohuangshan ophiolite are similar to tholeiitic rocks. They all have low TiO2 and high Al2O3 contents. Their light rare earth elements are slightly enriched, (La/Yb)N = 3.62–6.80, similar to the typical enriched mid-ocean ridge basalts. The mafic rocks display enrichments in large ion lithophile elements and depletions in high field strength elements, as well as significant Nb–Ta–Ti negative anomalies, similar to subduction-derived rocks. All these geochemical characteritics indicate that the Xiaohuangshan ophiolite would form in a subduction zone from a slightly enriched mantle source. Ion microprobes (SHRIMP) U–Pb dating were conducted on zircons from the basalt and gabbro. The weighted mean ages are 336.4 ± 4.1 Ma and 345 ± 14 Ma, which are considered as the crystallization ages of the basalt and gabbro, respectively. Together with other two units, the Dongqiyishan arc belt and the Yueyashan–Xichangjing ophiolite, the Xiaohuangshan ophiolite forms a Late Paleozoic arc-basin system, indicating that the Paleo-Asian Ocean did not close in the early Carboniferous. Based on the geochemical characteristics of adjacent geological bodies and their settings, the Xiaohuangshan ophiolite is considered as an indicator of a suture zone between the different epicontinental belts in the Beishan region. 相似文献
17.
《Chemie der Erde / Geochemistry》2016,76(2):299-308
The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al2O3 3.00–4.00 wt.%, Cr2O3 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al2O3 2.20–3.20 wt.% and Cr2O3 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al2O3 0.00–1.00 wt.% and Cr2O3 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre. 相似文献
18.
《Gondwana Research》2014,25(2):842-858
The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO2 and low K2O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al2O3/TiO2 ratios, but low TiO2 and SiO2 contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian. 相似文献
19.
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 FeOt (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 [ƐHf(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
20.
The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. 207Pb/206Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0 Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246 Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have εHf(t) values varying from − 0.7 to 7.6, which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs. εHf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (TDM = 2.9–3.4 Ga). In addition, the remarkably high εHf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO2 (46.5 to 52.8 wt.%) and high MgO (5.68 to 10.9 wt.%) contents, crust-like trace element features and low εNd(t) values (− 4.5 to − 0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic. 相似文献