蚌埠隆起老山片麻状二长花岗岩的成岩与变质特征及其地质意义

Diagenetic and Metamorphic Characteristics of the Laoshan Gneissic Monzogranite in the Bengbu Uplift and Its Tectonic Implications

蚌埠隆起是华北地块上中生代地质研究的一个十分难得的地质单元,它映现了中国东部最重要的两次构造运动(印支运动和燕山运动)所引起的岩浆作用和变质作用,其中,老山片麻状二长花岗岩就是一个代表性产物。该花岗岩的岩相学、地球化学以及锆石学的系统研究显示,它具有两个不同的岩石学特征:①它的斜长石斑晶的牌号(An_(14～18))、相对低硅高钙的主量元素、富Ba、Sr和贫Rb及无Eu负异常的微量元素等特征,以及它所含的岩浆锆石({211}锥面发育、较强的CL亮度及韵律环带构造、包含残核、正常的Hf含量)的U-Pb年龄(223.1 Ma)和ε_(Hf)(t)值(平均值为-14.2),表明它应该是印支运动的同造山花岗岩;②它的片麻状构造(斑晶矿物压扁拉长、各种交代—熔蚀结构)和变质结晶锆石({101}锥面发育、较弱的CL亮度、较高的Hf含量)的U-Pb年龄(156.2 Ma)和ε_(Hf)(t)值(平均值为-17.8),说明它在燕山运动中遭受了同造山变质作用。因此,老山片麻状二长花岗岩的存在,一方面使得大别造山带作为印支期陆陆碰撞造山带的岩石学记录完整,同时也制约了华南与华北两大板块之间碰撞挤压作用峰期的准确时间;另一方面,对燕山运动的起始时间(约170 Ma左右)和挤压高峰出现的时间(155±3 Ma)作出了合理的推断。这两方面的研究结果对于深入了解中国东部中生代的构造演化、岩石类型、成矿作用的现象及其机制具有深远的意义。

Objectives： The Bengbu Uplift has recorded multiple magmatic and metamorphic events related to two major tectonic movements（ i.e.,Indosinian and Yanshanian movements） in the eastern China,which makes it an ideal candidate for the study of Mesozoic tectonics of the North China Block.The Laoshan gneissic monzogranite on this massif is a representative sample.This paper reports an integrated petrographic,geochemical and zirconological study of the gneissic monzogranite to study its diagenetic and metamorphic characteristics and its tectonic implications.Methods： Petrographic observation,geochemical analysis,and zirconological study involving external morphology,internal structure,chemical composition,LA-ICPMS dating and Hf isotope analysis.Results： The Laoshan gneissic granite have two petrological properties：①The plagioclase phenocrysts has An%values of 14-18; the major element signature of the granite is characterized by relative high Ca but low Si concentrations,while the trace element signature is characterized by enrichment in Ba and Sr but depletion in Rb with no Eu negative anomalies; the magmatic zircons（ { 211 } developed,oscillatory zoned,high CL intensity,abundant relict cores,normal Hf contents） have a U-Pb age of 223.1 Ma and a mean ε（Hf）（t） value of-14.2;②The granite has a gneissosity defined by elongated mineral grains and various metasomatic—anatectic textures; the metamorphic zircons（ { 101} developed,weak CL intensity,high Hf contents） have a U-Pb age of 156.2 Ma and a mean ε（Hf）（t） value of-17.8.Conclusions： Based on the petrological,geochemical and zirconological results,the main conclusions can be drawn as follows：① The Laoshan monzogranite is a syn-orogenic granite related to the Indosinian movement.This finding completes the petrological record for the Dabie orogen as a continent—continent orogen,and its emplacement age can be used to accurately constrain the climax of the collision of the North China Block with South China Block.②The Laoshan gneissic monzogranite underwent syn-orogenic metamorphism during the Yanshanian movement.Its metamorphic age offers reasonable estimations of the onset time（ ca.170 Ma） of the Yanshanian movement and the climax time（ 155±3 Ma） of the Yanshanian compression.The above two findings may have profound implications for the in-depth understanding of the Mesozoic tectonic evolution,rock types,mineralization and its mechanism in the eastern China.