山东省沂水地区麻粒岩相变质作用演化及大地构造意义

The P T Trajectory and Its Tectonic Significance of Granulite Facies Metamorphism in the Yishui Area,Shandong Province

山东沂水麻粒岩相岩石出露于断裂带中，它是断裂带中最古老的岩石。主要岩石类型有孔兹岩（石榴石－石英－条纹长石－夕线石片麻岩）、紫苏辉石麻粒岩（紫苏辉石－斜长石－条纹长石－石榴石－石英片麻岩）、紫苏斜长麻粒岩、二辉斜长角闪岩（紫苏辉石－普通辉石－石榴石－斜长石－石英片麻岩）以及紫苏花岗岩等。根据矿物共生组合、变质反应、温压条件得出本区麻粒岩相岩石经历了五个阶段的变质作用：峰前阶段为角闪岩相变质作用，温压条件为Ｔ＝５７５℃～６００℃，Ｐ＝０．５９ＧＰａ；峰期阶段为麻粒岩相变质作用，温压条件为Ｔ＝８５０℃～９００℃，Ｐ＝０．９～０．９５ＧＰａ；峰后阶段亦为麻粒岩相变质作用，是峰期阶段麻粒岩经历了等压冷却的变质作用形成，其温压条件为Ｔ＝７５０℃，Ｐ＝０．９０ＧＰａ；第四阶段变质作用为角闪岩相的退变质作用，Ｔ＝５２５℃，Ｐ＝０．６５ＧＰａ；第五阶段为绿片岩相的退变质作用，Ｔ＝４００℃，Ｐ＝０．３５ＧＰａ，变质作用ＰＴｔ演化趋势为等压冷却（ＩＢＣ）的反时针轨迹。麻粒岩中反时针演化轨迹的形成及地壳热的扰动可能与板底大量幔源岩浆侵入有关，工作区存在基性岩浆岩，并且变质相呈热中心分布（麻粒岩相→高角闪岩相→角闪岩相）。沂水地区?

Rocks of granulitic facies in the Yishui area, Shandong Province are situated in the middle part of the Tancheng Lujiang Fault zone. They are the oldest rocks in the Fault zones. The principal rock types are khondalite (garnet quartz perthite sillimanite gneiss)、orthopyroxene granulite (orthopyroxene plagioclase perthite quartz garnet gneiss) and mafic granulites (orthopyroxene clinopyroxene garnet plagioclase quartz gneiss). In association of spinel garnet quartz silliminite perthite magnetite ilmentite, spinel is rimmed by sillmanite and garnet. There is no direct grain boundary contact between spinel and quartz. Garnet and sillimanite are also rimmed by coarse Fe Ti oxides. Garnet sillmanite corona is formed due to the equilibrium:Sp + Qz =Gt + Sill.In association of garnet orthopyroxene clinopyroxene plagioclase quartz, plagioclase is rimmed by garnet and quartz. Garnet and quartz coronas are formed due to the equilibrium:An + Hy =Qz + Gro + Alm.In association of orthopyroxene clinopyroxene biotite plagioclase quartz, orthopyroxene is rimmed by clinopyroxene. Clinopyroxene corona is formed due to the equilibrium:Hy + Pl =Py + Gt + Qz.In association of orthopyroxene clinopyroxene plagioclase quartz, amphibole is preserved as relics in clinopyroxene, it reflects the relics of earlier amphibolite facies of metamorphism.On the basis of reaction features and geothermobarometry, an anticlockwise P T trajectory was documented. The granulites underwent five episodes of metamorphism, The peak granulites facies metamorphsim took place at T =850℃～900℃ and P =0 90～0 95GPa; before the peak metamorphsim, there was an early amphibolite facies metamorphism with T =575℃～600℃ and P =0 59GPa; after the peak metamorphism, there was an isobaric cooling event having T =750℃, P =0 90GPa followed by a retrograde amphibolite facies metamorphism( T =525℃, P =0 65GPa) and a greenschist facies metamorphism ( T =400℃, P =0 35GPa).Perturbation of crustal geotherms and formation of anticlockwise granulite can be achieved by under plating of large volumes of mantle derived magmas (Bohlen,1987). Basic magmatic rocks are present in the studied area. Moreover, metamorphic facies are characteristic of thermo centers in their spatial distribution(granulite metamorphism→high amphibolite metamorphism→amphibolite metamorphism). Granulite facies metamorphism may be caused by the hot spot plume.This area was one of the thermal abnormal areas in Sino Korean plate in Archean, which had resulted in the extension within the Sino Korean plate and the rifting of which must have caused the development of the Tancheng Lujiang Fault.