大别超高压变质地体四道河地区岩石学研究

对四道河地区超高压变质岩剖面的研究分析显示,该剖面有3种岩石类型:榴辉岩类、片麻岩和面理化含榴花岗岩。榴辉岩具不同程度的退变质现象,呈透镜体状产出于斜长角闪岩、片麻岩和面理化含榴花岗岩中,原生矿物组合为石榴石、绿辉石、柯石英和金红石。榴辉岩退变为斜长角闪岩近于等化学系列;片麻岩在主量成分上与榴辉岩及其退变产物(斜长角闪岩)存在突变关系,但微量元素与榴辉岩有一定的相似性;面理化含榴花岗岩主量元素和微量元素地球化学特点为:富SiO2 、K2 O Na2 O和高价阳离子Ga、Y以及REE ,K2 O/Na2 O值低,贫Al、Ca、Mg、Ti、P ,结合构造环境、同位素及年代学资料分析,其应属于后碰撞造山A型花岗岩。基于以上认识推断:大陆板片俯冲至上地幔经历了超高压变质作用,表壳岩变质形成榴辉岩;当超高压变质岩石折返至中下地壳时发生了强烈的减压退变质作用形成斜长角闪岩,随后,与片麻岩及面理化含榴花岗岩一道从中下地壳向地表进一步折返,并一同经历了后期的变质变形作用。

Petrology of Sidaohe area in UHP metamorphic terrane, the Dabie Mountains

Based on a study of Sidaohe area in UHP metamorphic terrane within the Dabieshan Mountains, it is discovered that there exist three types of rock associations, namely eclogite, gneiss and foliated garnet-bearing granites. Eclogites underwent retrograde metamorphism of different degrees, and amphibolites seem to be retrograded products from eclogites. Gneiss is the country rock of eclogites, and there exist mutational relationships in major element components. Major and trace elements and REE in foliated garnet-bearing granites are characterized by enrichment of SiO-2, K-2O Na-2O, Ga, Y and REE components and depletion of K-2O/Na-2O, Al, Ca, Mg, Ti and P. Combined with Rb-Hf-Ta diagram and isotopic and geochronologic data, it is held that this type of granites belong to post-collisional A-type granites. Based on the above evidence, the authors have reached the conclusions that continental slab underwent UHP metamorphism and produced eclogites during its subduction to the depth of the upper mantle, that extensive decompression retrograde occurred in UHP metamorphic rocks and produced amphibolites when UHP slab migrated to the middle and lower crust, and that the UHP slab, together with gneiss and foliated garnet-bearing granites, further moved from the middle and lower crust to the surface and experienced subsequent metamorphism and deformation.