Platinum-group elemental and Re-Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China

The Late Middle-Permian Emeishan continental flood basalts (ECFB) lie between the Lower Permian Maokou Formation and Upper Permian Xuanwei Formation, being part of the ~260 Ma Emeishan Large Igneous Province (ELIP) in SW China [1]. Basaltic samples systematically collected in a cross section in Guizhou Province, the easternmost part of the ELIP [2], were analyzed for platinum-group elements (PGE) and Re-Os isotopes, in addition to major element oxides and trace elements. The 550-m thick sequence in the cross section includes 12 basaltic flows whose thicknesses range from 10 to 140 m. The majority of the basalts are high-Ti variety (TiO2=1.9-4.4 wt% and Ti/Y=400-700). In contrast to samples from Flows 9-12, those from Flows 1-8, the lower part of the sequence show depletion of Nb, Ta, P and Sr in the primitive mantle normalized trace elemental spider diagram and are relatively rich in Zr, La and Th, consistent with stronger crustal contamination. Gradual decrease of Zr/Nb, Th/Nb and Th/Ta ratios from the bottom to the top of the sequence, except Flow 9, indicate decreasing degrees of contamination. Os, Ir and Ru range from 0.01 to 0.23 ppb and are low relative to Rh, Pt and Pd which range from 0.2 to 1.1, 6 to 17 and 5 to 18 ppb, respectively. Different lava flows have different primitive mantle-normalized PGE patterns, although samples from the same flow show similar PGE patterns. Pd/Ir and Pt/Pd, Ni/Pd and Pd/Cr and Ni/Pd and Cu/Ir ratios show negative correlation, suggest the result of crystal fractionation. Rocks from Flows 1-8 derived from magma show lower degrees of partial melting than rocks of Flows 9- 12. The primitive mantle normalized pattern with clearly negative Ru anomaly for most of the Flows (except Flows 4 and 9) in the cross section can be interpreted by the fractionation of laurite or Os-Ir-Ru alloys in the primary magma and then coprecipitation with chromian spinel, olivine or sulfide fractionation. Thus the metal variations reflect variable extent of crystal fractionation and crustal contamination.published_or_final_versio

The ∼860-Ma, cordilleran-type guandaoshan dioritic pluton in the Yangtze Block, SW China: Implications for the origin of neoproterozoic magmatism

The tectonic setting of the widespread Neoproterozoic igneous rocks in South China has long been a matter of debate. Situated in the western margin of the Yangtze Block of South China, the Guandaoshan pluton is composed of diorite in the core and quartz diorite in the margin. Both types contain abundant mafic enclaves and mafic dikes. This pluton is dated at 858 ± 7 Ma by the SHRIMP zircon U-Pb method. The rocks have SiO 2 contents of 52.8-63.8 wt%, Al 2O 3 contents of 17.4-20.6 wt%, Na 2O contents of 2.14-3.74 wt%, and K 2O contents of 0.44-1.61 wt%. They are calcalkaline with an Al saturation index, A/CNK 1, and they have Fe numbers <0.8 (Fe 2O 3/[Fe 2O 3 + MgO]). Their total rare earth element (REE) contents range from 19 to 45 ppm, with concave chondrite-normalized patterns slightly depleted in middle REEs, indicating extensive amphibole fractionation. Their whole-rock ε Nd(t) values range from +3.9 to +5.1, and initial 87Sr/ 86Sr ratios range from 0.7028 to 0.7033. They have zircon ε Hf(t) values ranging from +11 to +17, with mean Hf model ages of around 860 Ma, clearly indicating a depleted-mantle source. In the primitive mantle-normalized trace element spider diagram, the rocks show pronounced enrichment of Rb, Ba, Th, and Pb and depletion of Nb, Ta, and Ti, a typical arc signature. The Guandaoshan pluton is an amphibole-rich, calc-alkaline, Cordilleran-type granitoid, a typical product of subduction-zone magmatism. The occurrence of this Neoproterozoic Cordilleran-type granitoid suggests that the western margin of the Yangtze Block was an Andean-type continental margin at that time, rather than mantle plume-related rifting setting. © 2008 by The University of Chicago. All rights reserved.published_or_final_versio

Neoproterozoic adakitic plutons and arc magmatism along the western margin of the Yangtze Block, South China

Neoproterozoic adakitic plutons that crop out along the western margin of the Yangtze Block (South China) from Kangding on the north to Panxi on the south provide constraints on the origin of the giant Jinningian magmatic event of South China. Representative plutons include the Xuelongbao (750 Ma), Datian (760 Ma), and Dajianshan intrusions. The latter two bodies consist mainly of granodiorite with relatively high SiO2 (51.0-73.4 wt%) and Mg#'s (0.36-0.55). They have fractionated rare earth element patterns, with (La/Yb)N ratios ranging from 2.6 to 101.8, and are characterized by high Sr (344-1018 ppm) and low Y (4.3-17.9 ppm), yielding Sr/Y ratios ranging from 27 to 111. On primitive mantle-normalized trace-element diagrams, these rocks show enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with positive Zr-Hf and negative Ti anomalies, consistent with an arc-related setting. They have relatively constant initial whole-rock 87Sr/86Sr ratios (0.704308-0.705068) and εNd values (+0.66 to -0.92). From their geochemistry, these plutons are interpreted to have formed in an arc environment. The parental magmas were generated from partial melts of a subducted oceanic slab that were modified by interaction with the overlying mantle wedge. Therefore, we conclude that the western margin of the Yangtze Block was an active magmatic arc during the Neoproterozoic. © 2007 by The University of Chicago. All rights reserved.published_or_final_versio

Melting of newly formed mafic crust for the formation of Neoproterozoic I-type granite in the Hannan region, South China

Neoproterozoic magmatism in the Hannan region at the northwestern margin of the Yangtze Block is characterized by numerous felsic plutons associated with minor mafic-ultramafic intrusions. The felsic plutons are either adakitic or normal-arc granitic in composition. The adakitic plutons are ∼735 Ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust. Among the normal-arc-related felsic plutons, the Tianpinghe pluton is the largest and has a SHRIMP zircon U-Pb age of 762 ± 4 Ma, older than the adakitic plutons in the region. Rocks from the Tianpinghe pluton have relatively high SiO2 (67.1-70.1 wt%) and K2O+ Na 2O (7.8-8.6 wt%) and relatively low MgO (0.7-1.3 wt%) and Al 2O3 contents (14.5-15.6 wt%), with Al2O 3/(CaO + K2O + Na2O) (A/CNK) values ranging from 0.95 to 1.08. They have arc-affinity trace-element compositions that are characterized by enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with strong positive Pb and negative Ti anomalies. They have a narrow range of εNd(t) values (+0.15 to - 1.76) and relatively high zircon eHf values (+0.6 to - 8.3). These geochemical features are typical of I-type granites. The rocks from the Tianpinghe pluton have relatively young single-stage and two-stage Hf model ages (1.01-1.31 and 1.31-2.01 Ga, respectively), suggesting that the pluton was generated by partial melting of newly formed basaltic rocks. On the basis of its arc-related geochemical affinity and its emplacement before voluminous adakitic magmatism but after mafic-ultramafic intrusions, the Tianpinghe pluton is considered to be Neoproterozoic arc granite formed during a period of crustal growth and reworking. Generation of the later adakitic plutons suggests that the crustal thickness increased to more than 50 km by mafic magma underplating. © 2009 by The University of Chicago. All rights reserved.published_or_final_versio

Late Paleoproterozoic sedimentary rock-hosted stratiform copper deposits in South China: their possible link to the supercontinent cycle

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Geochemistry and petrogenesis of the Yishak Volcanic Sequence, Kudi ophiolite, West Kunlun (NW China): Implications for the magmatic evolution in a subduction zone environment

The Yishak Volcanic Series (YVS) is part of the Late Cambrian Kudi ophiolite in the western Kunlun Mountains, NW China, which marks the oldest suture zone in the Tibetan Plateau. The YVS is a well-preserved sequence comprising five conformable units (A to E) of basalts to andesites, with mafic dykes in Units A to D. These rocks are sequentially characterized upward by geochemical features of N-MORB (Unit A), E-MORB (Unit B), island-arc basalt (Unit C), BABB (Unit D and mafic dykes) and boninite series rocks (Unit E), reflecting complex interactions among various source components. The evolution of magma compositions from MORB-dominated to boninite series rocks clearly indicates temporally increasing subduction signatures, which, in combination with the extensional tectonic background, suggests that the Kudi ophiolite most likely formed in a spontaneous nucleation of subduction zone. © Springer-Verlag 2005.postprin

Highly heterogeneous Late Mesozoic lithospheric mantle beneath the North China Craton: evidence from Sr–Nd–Pb isotopic systematics of mafic igneous rocks

The lithospheric mantle beneath the North China Craton changed dramatically in its geophysical and geochemical characteristics from Palaeozoic to Cenozoic times. This study uses samples of Mesozoic basalts and mafic intrusions from the North China Craton to investigate the nature of this mantle in Mesozoic times. Sr-Nd-Pb isotopic data demonstrate that the Late Mesozoic lithospheric mantle was extremely heterogeneous. In the central craton or the Luzhong region, it is slightly Sr-Nd isotopically enriched, beneath the Taihangshan region it has an EMI character (87Sr/86Sri = 0.7050-0.7066; εNd = -17--10), and beneath the Luxi-Jiaodong region, it possesses EM2-like characteristics (87Sr/86Sri up to 0.7114). Compositional variation with time is also apparent in the Mesozoic lithospheric mantle. Our data suggest that the old lithospheric mantle was modified during Mesozoic times by a silicic melt, where beneath the Luxi-Jiaodong region it was severely modified, but in the Luzhong and Taihangshan regions the effects were much less marked. The silicic melt may have been the product of partial melting of crustal materials brought into the mantle by the subducted slab during the formation of circum-cratonic orogenic belts. This Mesozoic mantle did not survive for a long time, and was replaced by a Cenozoic mantle with depleted geochemical characteristics. © 2004 Cambridge University Press.published_or_final_versio

The dynamothermal aureole of the Donqiao ophiolite (northern Tibet)

Metamorphic rocks found at the base of the Jurassic Donqiao ophiolite of northern Tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif. The rocks form a sequence some 8 m thick, varying from high-grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down. The mineral paragenesis is similar to other such aureoles, and indicates that temperatures in excess of 750°C may have been reached during metamorphism. The lack of high-pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone. Ar-Ar geochronology on amphibole separates provides dates of 175-180 Ma for the displacement of the ophiolite, significantly older than the age of emplacement estimated from stratigraphic relationships. The ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment.published_or_final_versio

Geochemistry of the Panzhihua Mafic Layered Intrusion (SW China): Petrogenetic Implications for its Giant V-Ti-Magnetite Deposit

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