The Mesozoic high-Sr granitoids in the northern marginal region of North China Craton: geochemistry and source region.
Liu HT, Sun SH, Liu JM and Zhai MG. 2002. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
The Mesozoic high Sr granitoids (HSRG) are widely distributed in the northern marginal region of the North China Craton and mainly consist of quartz diorites, granodiorites, quartz monzonites and adamellites, which are petrographically characterized by common occurrences of varying types of perthites. Based on analyses of 61 samples from 16 HSRG intrusions, they exhibit markedly high Na 2O and Al 2O 3 contents in their bulk compositions and can readily be classified into two groups: (1) low silica rocks with SiO 2 between 53%～60%, and (2) high silica ones with SiO 2 between 65%～73%, but only one sample is observed between 60%～65% SiO 2. Thus HSRG in the region display an apparent “bimodal” distribution. Majority of high silica rocks are high K calc alkaline, but the low silica rocks show a remarkable diversity in their affinity from low K island tholeiitic, through calc alkaline and high K calc alkaline, to shoshonitic. Their notable trace element geochemistry, including strongly enriched strontium, strongly depleted heavy rare earth elements (HREE) and nearly absence of negative europium anomalies, thus making them distinguishable from other granites occurred in the same region. By detailed comparison between HSRG and the partial melts generated experimentally from dehydration melting of various rocks commonly occurred within continental crust, it is suggested that the low silica rocks from HSRG were derived from partial melting of basaltic amphibolites, but the high silica rocks were generated from partial melting of intermediate to acid igneous protoliths, both under high pressure (assumedly ≥ 1.5 GPa) and high temperature (assumedly 850～900 ℃) conditions. The partial melts extracted from the residual assemblages composed mainly of garnet + clinopyroxene ± orthopyroxene ± amphibole ± quartz would unequivocally display the major and trace element geochemistry as the high Sr granitoids do. The HREE depletions of HSRG could largely be ascribed to the residual garnet as exhibited by dehydration melting experiments, and both high Sr abundance and absence of negative Eu anomalies could be largely contributed by nearly completely fusing of feldspar components of the protoliths into partial melts. Furthermore, they also indicate no significant plagioclase fractionation occurred in between from melt segregation to magma emplacement. This implies that the HSRG were originated directly from mafic and/or intermediate acid igneous protoliths in the overthickened continental lower crusts and may generally represent the primary granitic magmas with little differentiation before the onset of major phase of crystallization. The residual solid assemblages of granulite and/or eclogite facies are suggested to become new constituents of the continental lower crust below where HSRG emplaced, or to be delaminated down to the upper mantle by their density inversion.
【Fund】： 中国科学院资源环境领域知识创新工程重大项目 ( KZCX1-0 7)研究成果
【CateGory Index】： P588.121
【CateGory Index】： P588.121