摘要
Voluminous granitoids are widely distributed in the Langshan region,northeast of the Alxa block,and record the evolutionary processes of the southern Central Asian Orogenic Belt.The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex.Early Carboniferous zircon LA-ICP MS U-Pb ages were from 327 Ma to 346 Ma.The Dabashan pluton can be classified as monzogranite and syenogranite,and exhibits high K2 O contents and K2 O/Na2 O ratios,which reveal a high-K calc-alkaline nature.The samples display strongly fractionated REE patterns,and are enriched in large ion lithophile elements(LILE)relative to high field strength elements(HFSE).The Dabashan plutons display unusually high Ba(823–2817 ppm)and Sr(166–520 ppm)contents and K/Rb ratios(315–627),but low Rb/Ba ratios(0.02–0.14),and exhibit fertile zircon Hf isotopic compositions[εHf(t)=-14 to-20],which are comparable to those of typical high Ba–Sr granitoids.Based on the geochemical compositions of the samples,we suggest that subducted sediments and ancient crustal materials both played important roles in their generation.Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment-related melts with residual garnet in the source,which caused partial melting of ancient lower crust.Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths.The resultant magmas experienced some degree of fractional crystallization during their ascent.Given these geochemical characteristics,together with regional tectonic,magmatic,and structure analysis data,an active continental margin environment is proposed for the generation of these rocks.
Voluminous granitoids are widely distributed in the Langshan region, northeast of the Alxa block, and record the evolutionary processes of the southern Central Asian Orogenic Belt. The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex. Early Carboniferous zircon LA-ICP MS U-Pb ages were from 327 Ma to 346 Ma.The Dabashan pluton can be classified as monzogranite and syenogranite, and exhibits high K2 O contents and K2 O/Na2 O ratios, which reveal a high-K calc-alkaline nature. The samples display strongly fractionated REE patterns, and are enriched in large ion lithophile elements(LILE) relative to high field strength elements(HFSE). The Dabashan plutons display unusually high Ba(823–2817 ppm) and Sr(166–520 ppm) contents and K/Rb ratios(315–627), but low Rb/Ba ratios(0.02–0.14), and exhibit fertile zircon Hf isotopic compositions [εHf(t)=-14 to-20], which are comparable to those of typical high Ba–Sr granitoids. Based on the geochemical compositions of the samples, we suggest that subducted sediments and ancient crustal materials both played important roles in their generation. Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment-related melts with residual garnet in the source,which caused partial melting of ancient lower crust. Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths. The resultant magmas experienced some degree of fractional crystallization during their ascent. Given these geochemical characteristics, together with regional tectonic, magmatic, and structure analysis data,an active continental margin environment is proposed for the generation of these rocks.
基金
supported by the National Key Research and Development Program of China from the Ministry of ScienceandTechnologyofChina(No.2017YFC0601301)
the National Natural Science Foundation of China(41502214,41230207 and 41572190)
the Outlay Research Fund of Institute of Geology,Chinese Academy of Geological Sciences(J1706)
the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(XDB18020203)
the CPSF-CAS Joint Foundation for Excellent Postdoctoral Fellows(Grant no.2015LH0049)
the China Postdoctoral Foundation funded project(2016M590990)
the Key Research Program of Frontier Sciences,CAS(QYZDJSSW-SYS012)
China Geological Survey(12120115069601)
