中国东南新生代玄武岩中麻粒岩相捕虏体的新发现及其意义

本文报道了在中国东南地区女山、桂子山、西垄和麒麟等地新生代玄武岩中新发现的麻粒岩相捕虏体。研究显示女山存在多种岩石类型的捕虏体，其中本次研究新发现的石榴子石麻粒岩与二辉麻粒岩在矿物化学和形成条件上明显不同。石榴子石麻粒岩的次透辉石（富Al2O3，Na2O)、紫苏辉石（富Al2O3) 形成于较深（＞40km)的下地壳，厚的地壳和较低的地温梯度（20－24℃/km)表明当时女山处于相对稳定的克拉通环境；女山二辉麻粒岩的次透辉石和紫苏辉石（贫Al2O3,Na2O)形成于较高地温梯度（31－34℃/km）的构造背景，指示当时女山处于活动大陆边缘或裂谷环境。复杂的岩石类型和不同的形成条件表明女山的下地壳是由多期岩浆活动的产物组成。广东麒麟和雷州的二辉麻粒岩的次透辉石以低Al2O3和低Na2O为特征，温压估算显示它们形成于较浅部（23－27km)和高地温梯度的地质背景，指示当时华南处在强烈的拉张减薄的构造环境。浙江西垄和新昌的麻粒岩捕虏体的矿物学特征和形成条件介于女山和广东的麻粒岩捕虏体之间。女山麻粒岩捕虏体的岩石组合和形成条件与华北太古代麻粒岩地体和汉诺坝麻粒岩捕虏体相似，而与华南的明显不同，表明女山的下地壳隶属于华北板块。在华南沿海从北到南基性麻粒岩的发现，显示中生代基性岩浆的底侵作用普遍存在。

Discovery and Implications of Granulite Facies Xenoliths from Some Cenozoic Basalts, SE China

This study presents the petrological and mineralogical characteristics of granulite xenoliths from Cenozoic basalts in Nushan,Guizishan,Xilong and Qilin,Southeastern China.Garnet granulite was first discovered in Nushan Cenozoic basalt.It is significantly different from two-pyroxene granulite in mineral compositions and formation condition.Clinopyroxenes from the garnet granulites have high Al203 and Na20,and orthopyroxenes are rich in Al203. Garnet granulites formed in deep lower crust(> 40 km)and lower geotherm (19～ 25E ／km),suggesting that Nushan was in relatively stable craton at that time.Two granulite samples from Nushan contain the clinopyroxene and orthopyroxene with lower Al203 and Na20 contents.They formed in the tectonic setting with higher geotherm (31～34℃ /km),which probably was an extensional environment in continental margin or rift.Complex granulite facies xenolith types and distinct formation conditions indicate that the lower crust beneath Nushan was constructed by multi-periodic magmatism in different tectonic settings. Lower A1203 and Na20 contents characterize the salites of two-pyroxene granulites from Qilin and Leizhou basalts,Guangdong Province.T-P estimations indicate that these granulites recrystallized at shallower level(23～27km)and high geotherm environment.It is suggested that South China was in the intense extension setting during that time.Granulite xenoliths from Xilong and Xinchang of ZheJiang Province were between those of Nushan and Qilin—Leizhou in mineral composition and recrystallization condition.The rock assemblage and formation condition of Nushan granulite facies xenoliths are similar to those from Archean granulite terrene and Hannuoba granulite xenoliths in North China,and markedly different from those in South China.It is suggested that the Nushan lower crust component should be a part of North China lower crust.In South China many mafic granulites were found from north to south, showing Mesozoic mafic magma underplating was widespread under the coastal region of South China.