浙西南遂昌—大柘地区八都岩群印支期变质变形序列

Indosinian Metamorphic Deformation Sequence of Badu Group in Suichang-Dazhe Region,Southwestern Zhejiang

浙西南遂昌-大柘地区八都岩群在印支期变质事件影响下发生变质变形,通过详细野外调查和岩相学研究,可将其划分为3期变质变形序列:S1变形期,NW向片麻理记录的残留紧闭褶皱,共生矿物组合为石榴子石变斑晶及其内部定向分布的包裹体矿物,石榴子石+黑云母+石英(泥质)和石榴子石+角闪石+斜长石+石英(长英质);S2变形期,区域性宽缓褶皱及NE向缓倾透入性片麻理,共生矿物组合为石榴子石变斑晶及定向分布的基质矿物,矽线石+石榴子石+黑云母+石英+斜长石±钾长石(泥质)和石榴子石+钾长石+斜长石+黑云母+石英(长英质);S3变形期,NE向陡倾透入性片麻理及韧脆性断裂大部分被花岗斑岩脉填充,共生矿物组合为石榴子石变斑晶及其周围退变矿物,石榴子石+矽线石+堇青石+斜长石+黑云母+石英±钾长石(泥质)和角闪石+斜长石+黑云母+钛铁矿(长英质)。结合前人研究成果,八都岩群印支期变质事件峰期变质程度达到麻粒岩相,显示顺时针近等温降压(ITD)型的p-T演化轨迹,S1-S3变质变形反映出从俯冲碰撞到快速折返冷却的演化过程,伴随S3同期侵位的花岗斑岩锆石U-Pb定年结果,将该演化过程完成时间约束在229.7 Ma,可能是浙西南地区对印支期古特提斯洋域内印支华南华北板块之间俯冲碰撞过程的响应。

The Badu Group in Suichang Dazhe region of southwestern Zhejiang underwentmetamorphism and deformation under the influence of Indosinian metamorphic event.Through adetailed field investigation and lithofacies study,it can be divided into three metamorphism-deformationsequences:for Si deformation period,the residual tight folds are recorded by NW-trending gneisses andthe symbiotic mineral assemblages are garnet metamorphic phenocrysts with inclusion minerals ofgarnet+biotite+quartz(argillaceous)and garnet+amphibole+plagioclase+quartz(feldspar)distributed directionally;for S2 deformation period,the regional wide-gentle folds and NE-trendinggently inclined permeable gneisses are developed,and the symbiotic mineral assemblages are garnetmetamorphic phenocrysts and matrix minerals of garnet+biotite+silica+quartz+K feldspar+plagioclase(argillaceous)and garnet+K-feldspar+plagioclase+biotite+quartz(feldspar);for S3 deformation period,the NE-trending steeply inclined permeable gneisses and ductile-brittle fractures aremostly filled by granite porphyry veins,and the symbiotic mineral assemblages are garnet metamorphicphenocrysts and their retrograde edge minerals of garnet+cordierite+sillimanite+plagioclase+biotite+quartz+K-feldspar(argillaceous)and amphibole+plagioclase+biotite+ilm enite(feldspar).Combinedwith the previous research results,the peak metamorphic degree of Indosinian metamorphic event ofBadu Group reached granulite facies,showing a clockwise near isothermal decompression(ITD)type p-T evolution track.The Si-S3 metamorphism and deformation reflect the evolution from subductioncollision to rapid re-cooling.The zircon U-Pb dating results of granite porphyry restrict the completiontime of the evolution process to 229.7 Ma which may be the response of southwestern Zhejiang to thesubduction-collision between the Indosinian-South China-North China plate to the Paleo-Tethyan Oceanduring Indosinian.