三江盆地北缘晚白垩世花岗质岩石的年代学、地球化学及其构造意义

Geochronology and geochemistry of the Late Cretaceous granitoids in the northern margin of the Sanjiang basin, NE China and its tectonic implications.

本文报道了黑龙江东部三江盆地北缘花岗质岩石的锆石LA-ICP-MSU-Pb定年结果、全岩地球化学和锆石Hf同位素资料,以确定该区花岗质岩石的形成时代、源区性质及其构造属性。花岗质岩石中的锆石多呈自形-半自形晶,振荡环带发育,Th/U比值=0.32～1.09,指示其岩浆成因。对岩浆锆石的定年结果表明,这些花岗质岩石均形成于晚白垩世(88.9～95.1Ma),而非前人认为的晚印支期。该期花岗质岩石均为含角闪石黑云母花岗闪长岩,它们的SiO2含量主要在63.9%～68.1%之间,Na2O=3.03%～4.25%,K2O=2.28%～3.27%,Na2O/K2O=1.06～1.73,A/CNK=0.94～1.08;稀土配分模式均呈右倾型,除了个别样品外,它们的LREE/HREE=8.13～15.5,(La/Yb)N=8.61～21.0,δEu=0.82～1.11;在微量元素蛛网图上表现为大离子亲石元素(Rb、Ba、K)的富集和高场强元素(Nb、Ta、Zr、Hf)及P的亏损;锆石εHf(t)值介于+4.44～+9.00之间,二阶段Hf模式年龄(tDM2)介于583～875Ma之间。上述特征表明,该期花岗质岩石具有准铝质-弱过铝质的高钾-中钾钙碱性I型花岗岩的成因类型,源区岩石为新增生的陆壳物质,岩浆起源于深部陆壳基性火成岩的部分熔融。结合同时代火成岩的组合特征和区域构造演化历史,认为该区晚白垩世花岗质岩石形成于古太平洋板块向东亚大陆下俯冲所产生的活动大陆边缘环境,该期岩浆事件标志着古太平洋板块正向(北西向)俯冲作用的开始。

This paper reports zircon U-Pb dating, geochemical and Hf isotope data of the Late Cretaceous granitoids from the northern margin of the Sanjiang basin, NE China, with the aim of constraining formation time, magma source, and tectonic setting of these granitoids. Zircons from these granitoids are euhedral-subhedral in shape, display fine-scale oscillatory growth zoning, and have Th/U ratios of 0.32～1.09, indicating a magmatic origin. The dating results indicate that these granitoids formed in the Late Cretaceous （88.9～95.1Ma） rather than previously believed Late Indo-Chinese epoch. The Late Cretaceous granitoids consist of hornblende-bearing biotite granodiorite, have SiO2=63.9%～68.1%, Na2O=3.03%～4.25%, K2O=2.28%～3.27%, Na2O/K2O=1.06～1.73, A/CNK=0.94～1.08. Their REE patterns are right-leaned and characterized by LREE/HREE=8.13～15.5, （La/Yb）N=8.61～1.0, and δEu=0.82～1.11. These granitoids are relatively enriched in LREE and LILE （such as Rb, Ba, K）, and depleted in heavy rare earth elements （HREEs） and high field strength elements （HFSEs） such as Nb, Ta, and Ti, as well as P. The εHf（t） values of zircons from these granitoids range from ＋4.44 to ＋9.00, their tDM2 vary from 583Ma to 875Ma. Taken together, it is suggested that these granitoids chemically belong to metaluminous-weak peraluminous and middle-to high-K calc-alkaline I type ones, and that their primary magma could be derived from partial melting of mafic materials within lower crust. Combined with the coeval igneous rock associations and regional tectonic evolution, we conclude that the Late Cretaceous granitoids occurred in an active continental margin setting related to the subduction of the Paleo-pacific plate beneath the East Asia continent, and this episode of magmatic event would mark the beginning of orthogonal （northwestward） subduction of the Paleo-pacific plate.