SHRIMP zircon U-Pb dating, geochemistry, Sr-Nd-Hf isotopic analysis of the Yingqian intrusion in Jiangxi Province, South China and its geological significance
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摘要: 赣南营前岩体由早期似斑状花岗闪长岩和晚期细粒黑云母花岗岩组成,在前者中普遍发育暗色基性包体。SHRIMP锆石U-Pb测年显示似斑状花岗闪长岩为172.2±3.1Ma,细粒黑云母花岗岩为168±3Ma,包体为167.6±3.3Ma。所有的岩石均为钙碱性系列,稀土元素显示LREE富集,HREE亏损,Eu负异常并不明显的特征,Nb-Ta负异常显示壳源物质的特征。晚期细粒黑云母花岗岩、早期似斑状花岗闪长岩、暗色包体的(87Sr/86Sr)i和εNd(t)值分别为0.70885~0.70965、0.71058~1.46393、0.70788~0.70923和-6.38~-5.31、-6.51~-5.17、-5.47~-4.31; 锆石εHf(t)值分别为-12.4~-5.7、-9.2~-4.2、-9.4~+0.1。结合岩相学、地球化学研究,初步认为营前岩体的两期花岗质岩石源于地壳重熔,地幔底侵导致地壳熔融形成营前岩体,基性岩浆加入到未冷却的酸性岩浆中形成了目前所见到的暗色包体。结合区域上的研究资料,认为中侏罗世期间,沿"十杭"断裂带及与其相平行的一些深断裂发生了板块的撕裂,这些缝合带是岩石圈相对薄弱的地带,有利于幔源物质底侵上升以及壳幔相互作用。Abstract: The Yingqian intrusion is located in southern Jiangxi Province of the East Nanling region, and consists of granitoids of two periods of granitoids, mega-porphyritic granodiorites and fine-grained granites. There are a lot of melanic monzonitic enclaves in mega-porphyritic granodiorites. SHRIMP zircon U-Pb dating gives the ages of 172.2±3.1Ma for the earlier granodiorites, 167.6±3.3Ma for enclaves, 168±3Ma for later granites, indicating that the Yingqian intrusion is a Mid-Jurassic magmatic pluton. All of the rocks fall into the calc-alkaline category. Their whole rock trace elements demonstrate total LREE enrichment without significant Eu anomalies and obvious fractionation between LREE and HREE. LILE enrichment and HFSE depletion, and significantly negative Nb-Ta anomalies characterized the primitive mantle-normalized trace element spidergrams of the rocks. The (87Sr/86Sr)i ratios of the fine-grained granites, mega-porphyritic granodiorites and enclaves are 0.70885~0.70965, 0.71058~1.46393, 0.70788~0.70923 with εNd(t) values from -6.38~-5.31, -6.51~-5.17 to -5.47~-4.31 respectively. The εHf(t) values are -12.4~-5.7, -9.2~-4.2, -9.4~+0.1 respectively. These data indicate that the Yingqian intrusion were result of partial melting of ancient crustal materials, interacted with mantle-derived magmas. The Mid-Jurassic magmatism along the "Shi-Hang" belt formed in a continental crustal thinning setting, which is a favorable event of interaction between crustal and mantle.
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