Qi-Qin Accretionary Belt in Central China Orogen： accretion by trench jam of oceanic plateau and formation of intra-oceanic arc in the Early Paleozoic Qin-Qi-Kun Ocean

Most orogenic belts have experienced a complex accretionary process with multiple episodes of sea?oor subduction and trench retreat.This accretionary process is important in continenta development and growth[1,2].Three giant orogens extend in China,e.g.,the Central Asian Orogen in the north,the Central China Orogen in the middle and the Himalayan Orogen in the southwest.They are keys for the formation of the Eurasian continent(Fig.1a).The Central China Orogen

Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen,NW China:Evidence for decompression melting of oceanic slab

A trondhjemitic body occurs in the Aoyougou area,the western part of the North Qilian orogen.It is geochemically characterized by high SiO 2,Na 2 O (high Na/K),elevated Sr/Y and (La/Yb) N,positive Sr anomaly,relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs),and depleted Nb,Ta,Ti,resembling the high-silica adakite.Zircon U-Pb SHRIMP dating yields a weighted mean age of 438±3 Ma.This age is significantly younger than eclogitization ages of 460-490 Ma in the North Qilian orogen,suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean.Whole-rock Sr and Nd isotopic analyses give initial ratios of I Sr =0.7044 0.7047 and Nd (t)=3.0 4.1,indicating that they are derived from partial melting of the juvenile oceanic crust.In view of the tectonic evolution of the North Qilian orogen,the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of ～60 km.

First discovery of coesite in eclogite from East Kunlun, northwest China

Ultrahigh-pressure (UHP)metamorphism plays important roles in continental dynamics.Understanding of this process has been enhanced by the discovery of coesite and micro-diamond from supracrustal rocks [1].Such UHP mineral inclusions provide unequivocal evidence that the least dense part of the lithosphere can be buried to 100-150km or more,and then returned to the surface.Coesite,an ultrahigh-pressure polymorph of SiO2stable above 2.7GPa at metamorphic temperatures,is one of the most important index minerals for UHP metamomhism [1,2].

Exotic origin of the Chinese continental shelf

The history of western Pacific subduction and its relation to eastern China are big issues for geological investigations[1,2].Many models have been proposed for interpreting the tectonic scenarios of eastern China since the Mesozoic,but most have centered on the lithosphere destruction of the North China Craton[3–6]with increasing emphasis on the effect

Application of microprobe-based flank method analysis of Fe^3＋ in garnet ofNorth Qilian eclogite and its geological implication

A newly developed microprobe-based methodology(the Flank Method)for Fe^(3+)/RFe quantification has been successfully applied to some natural garnets from the North Qilian eclogites by JEOL JXA-8100microprobe at Peking University.The results demonstrated an obvious discrepancy in comparison with the outcomes by conventional stoichiometric calculations.This methodology allows to measure theFe^(3+)/Rmeasurement of Fe^(3+)According to the compositional zonation displayed in the studied eclogitic garnets from North Qilian,a prograde metamorphic°C to 22 kbar,600°C was reconstructed.More interestingly,the measured Fe^(3+)/RFe ratios in garnets decreasing from core to rim may probably imply that the oxygen fugacity(f O_2)declines with the depth of the subduction zone.