Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis

The Altay orogenic belt of Xinjiang in NW China represents one of the important sites of juvenile crustal growth during the Phanerozoic. However, some important issues, e.g., tectonic evolution and petrogenesis, still remain controversial. The picrites in the south margin of the Altay orogenic belt were discovered in the lower part of marine volcanic-sedimentary sequences of the northwest-striking Middle Devonian Beitashan Formation (Fm.), which consists chiefly of intermediate-basic volcanic rocks intercalated minor carbonate, siltstone and siliceous rocks. The picrites are usually highly porphyritic, and contain abundant forsteritic olivine phenocrysts with minor clinopyroxene distributed in the groundmass, which consist of olivine, clinopyroxene and plagioclase with minor Fe-Ti oxides. The MgO contents of the picrites range from 14 wt% to 22 wt% with Mg# (atomic Mg/(Mg+Fe) ratio) of 0.75-0.80. They are characterized by slightly negative Ti anomalies, remarkably negative Nb and Ta anomalies and slightly positive P and Sm anomalies with the similar abundances of HFSE as MORB on the MORB-normalized trace element patterns, all of which characterize typical island arc magmas. In combing with the southwestward migration of the magmas of the Beitashan Fm., we propose that the magmas may result from the southwestward subduction of Junggar ocean plate. The Zr/Nb ratios (23-66) of both picrites and basalts resemble the MORB (10-66), suggesting that they were derived from the MORB-like sources. However, the basalts and picrites display some distinguishable element ratios and REE patterns, e.g., Ti/V (23-35) and Zr/Sm (18-23) ratios of basalts are higher than those of picrites (14-17 and 14-15 respectively), and the basalts display flat-type REE-chondrite patterns whereas the picrites are characterized by lower total REE concentrations ((26-34)×10^-6) and slight enrichment of light REE. These distinguished geochemical characteristics could be interpreted by different partial melting degrees and mantle sources, i.e., the basalts were generated by lower partial melting of amphibole-bearing spinel peridotite which was metasomatized by fluids released from subducted oceanic crust, and the picrites were resulted from the higher degree of partial melting of metasomatized garnet peridotite under high temperature. In contrast, the andesite with significant LREE and LILE enrichment may be resulted from the partial melting of eclogites.

Platinum-Group Element Geochemical Characteristics of the Picrites and High-Ti Basalts in the Binchuan Area,Yunnan Province

The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part. The picrites have relatively higher platinum- group element （PGE） contents （PGE=16.3-28.2 ppb）, with high Cu/Zr and Pd/Zr ratios, and low S contents （5.03-16.9 ppm）, indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province （ELIP） mantle source. The slightly high Cu/Pd ratios （11 000-24 000） relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high-Ti basalts exhibit a wider range （~PGE=0.517-30.8 ppb）. The samples in the middle and upper parts are depleted in PGE and have ~Nd （260 Ma） ratios ranging from -2.8 to -2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides, and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts. The samples in this area show Pt- Pd type primitive mantle-normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle （Pd/Ir=l）, indicating that the obvious differentiation between Ir-group platinum- group elements （IPGE） and Pd-group platinum-group elements （PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle （Pd/Pt=0.55）, showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle; whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys, coprecipitating with Ir-Ru-Os alloys. Some high-Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.

Petrology, Mineralogy and Geochemistry of the Emeishan Continental Flood Basalts, SW China: Evidence for Activity of Mantle Plumes

Electronic microprobe analyses for olivine, clinopyroxene and Cr-spinel in picrites, which we have discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg, and that Cr-spinel is rich in Cr. Based on the olivine-melt equilibrium, the primary parental melt compositions are calculated. The high-Mg olivine-hosted picrite can be regarded as parental melt. Thus, the melting temperature and pressure are estimated: T=1600℃ and P=4.5 GPa. It suggests that the picrites are connected with the activity of mantle plumes. Their major element composition is comparable to many other CFBs by their high Fe8, (CaO/Al2O3)8 and low Na8, indicating a high pressure. All rocks display a similar chondrite-normalized REE patterns, i.e., enrichment of LREE, relative depletion of HFSE and absence of negative Nb and Ta but depletion in P and K. Some incompatible element ratios, such as La/Ta, La/ Sm, (La/Nb)PM, (Th/Ta)PM, are in a limited range, show that they were derived from the mantle plume, and there was no or little crustal contamination during magma ascent en route to the surface. They were generated by 7% partial melting of garnet peridotite. The axis of the plume might be located beneath Lijiang Town, Yunnan province.

Geochemical characteristics and geological significance of picritic rocks in Permian Tiaohu Formation from Santanghu Basin, Xinjiang, NW China

1 Introduction Santanghu Basin is located between the Armantai and Karamaili suture zone,at the junction of the Siberia,Kazakhstan and Tarim plates(Chen and Jahn,2004;Xiao et al.,2008).As an important part of the Central Asian

Geochemical characteristics and tectonic setting of the Middle Permian Tiaohu Formation mafic-ultramafic rocks of Santanghu area, Xinjiang, Northwest China

Santanghu area in northeastern Xinjiang region of Northwest China is an important component of the Central Asian Orogenic Belt(CAOB), in which the dynamic mechanism of Permian magmatism is controversial. In Santanghu area is exposed a thick succession of the Middle Permian basalts, including a small amount of picritic basalts and andesites, known as the Tiaohu Formation. The picritic basalts contain cumulate olivine, and have whole-rock Mg# up to 0.68–0.77; the basalts exhibit porphyritic or doleritic textures, and have relatively low Mg# of 0.41–0.54, typical of evolved magmas. The mafic-ultramafic rocks of the Tiaohu Formation are slightly enriched in Light Rare Earth Elements(LREEs), and exhibit negative Nb and Ti anomalies. They also have high Ti O2 content, and Nb/Y and Zr/Yb ratios greater than those of island arc volcanic rocks. Relatively low initial Sr isotopic ratios and high positive εNd(t) and εHf(t) values argue against contamination by ancient continental crust, and suggest formation of the Tiaohu Formation by partial melting of relatively refractory depleted lithospheric mantle that underwent metasomatism and extraction by fluid from the subducted slab. In addition, up to 38% olivine in picritic basalts indicates high-degree partial melting of lithospheric mantle, and the underlying Lucaogou Formation contains fragments of ultra-alkaline magmatic rocks that originated in the deep mantle. These observations imply wide-spread underplating in Santanghu area, which may have been associated with a mantle plume.