华北克拉通北缘晚志留世末-早泥盆世钾质碱性岩的成因及对区域构造演化的限定

华北克拉通北缘晚志留世末-早泥盆世碱性岩的成因研究可以为古生代古亚洲洋与华北克拉通的相互作用过程提供重要信息。本文以内蒙古达茂旗黄合少正长岩为研究对象,开展了锆石U-Pb年代学、矿物化学、地球化学和Sr-Nd同位素的研究,并综合同带其他代表性岩体的岩石矿物及元素特征,探讨了华北克拉通北缘钾质碱性岩的岩浆体系性质、源区特征及构造动力学背景。黄合少正长岩的主要岩石类型为碱性长石正长岩,锆石SHRIMP U-Pb年龄为417.6±4.7 Ma,为晚志留世末期岩浆作用的产物;主要组成矿物为霓辉石和钾长石(An_0Ab_(2～5)Or_(95～98))及少量钠长石(An_0Ab_(95～100)Or_(0～5)),可见霓辉石内部部分转化为蓝透闪石或镁钠闪石;黄合少正长岩具有较高的全碱含量(Na_2O+K_2O=12.79%～15.30%)和里特曼指数(8.48～11.09),较高的K_2O含量(8.66%～13.78%)和K_2O/Na_2O值(2.10～9.07),指示黄合少正长岩属于钾质-超钾质碱性岩系列;较高的Sr含量和富集放射性成因的Sr同位素组成(初始~(87)Sr/~(86)Sr值为0.704 1～0.706 1),显示岩体起源于被俯冲沉积物交代过的富集岩石圈地幔。华北克拉通北缘晚志留世末-早泥盆世钾质碱性岩的发育,指示早古生代期间发育古亚洲洋板片向华北克拉通下的俯冲,俯冲相关物质对华北克拉通岩石圈地幔进行交代,形成含金云母/角闪石等交代矿物的易熔地幔;晚志留世末-早泥盆世时,由于微陆块的拼贴碰撞,华北克拉通北缘暂时处于碰撞后伸展演化阶段。

Petrogenesis of the Late Silurian to Early Devonian potassic alkaline rocks on the northern margin of the North China Craton and their constraints on the tectonic evolution

The Late Silurian to Early Devonian potassic alkaline rocks on the northern margin of the North China Craton (NCC) is a key geological unit to understand the interaction processes of the Paleo-Asian Ocean and the NCC in the Paleozoic. In this paper, we report new zircon U-Pb age, mineral chemistry, whole-rock geochemistry and Sr-Nd isotopic compositions from the Late Silurian potassic Huangheshao (HHS) syenties on the northern margin of the NCC, and discuss its petrogenesis. Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating has revealed a crystallization age of 417.6±4.7 Ma for the HHS pluton. Alkali-feldspar syenite is the main rock type; Aegirine-augite and sanidine (An₀Ab_2~5Or_95~98) are the major minerals, with subordinate albite (An₀Ab_95~100Or_0~5). Aegirine-augites locally transferred into winchite or magnesio-riebeckite. The HHS syenites have high total alkali contents (Na₂O+K₂O=12.79%~15.30%),high Rittmann index (σ=8.48~11.09), high K₂O contents (8.66%~13.78%), and high K₂O/Na₂O ratios (2.10~9.07), suggesting their alkaline, potassic-ultrapotassic affinity. High Sr abundances and radiogenic Sr isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr=0.704 1~0.706 1) argue for an enriched subcontinental lithospheric mantle (SCLM) source for the HHS magmas,which was previously metasomatized by melts/fluids from the subducted sediments on the down-going oceanic crust. The development of the Late Silurian to Early Devonian potassic alkaline rocks on the northern margin of the NCC indicates that the Paleo-Asian oceanic slab had subducted beneath the NCC in the Early Pleozoic, and the subducted materials metasomatised the SCLM, forming a phlogopite/K-amphibole-bearing, fertile mantle source. During the end of the Late Silurian to Early Devonian, due to the accretion or collision of a microcontinent, the northern margin the NCC has entered into a temporary, post-collisional extension tectonic setting.