湘东邓阜仙－锡田印支期花岗岩体的侵位机制

Emplacement Mechanism of Indosinian Dengfuxian-Xitian Granite Pluton in Eastern Hunan,South China

印支期似斑状花岗岩构成了湘东中生代邓阜仙-锡田岩体的主体,其内发育两种类型的叶理构造,即流面和构造成因的片麻理。二者的标志性区别在于长石(斜长石和钾长石)斑晶在前者呈自形的短柱状,没有任何可见的韧性变形,而在后者呈椭圆状,明显被压扁拉长。流面基本上围绕岩体边界呈面状分布,整个侵入体从外到内依次可划分出流面不发育、欠发育和发育三个相带,对应着由早至晚的三次岩浆脉动侵位。该期花岗岩侵位机制总体上类似于气球膨胀模型。与典型气球膨胀岩体不同的是,由于岩浆供给量在该期最后一次脉动侵位时的急剧减少,虽然流速增大,但对外挤压减弱,造成岩石应变强度从外到内趋向增加,而不是递减。构造成因的片麻理局部出现,走向总体上呈NE向,表明该花岗岩在准固态甚至固态下遭受到NW-SE向区域挤压作用。

In an Indosinian porphyritic granite that comprises a majority of the Mesozoic Dengfuxian-Xitian granite batholith in eastern Hunan, central South China, there exist two types of foliations, flow foliation and tectonic gneissosity. Their discrimination hinges on the configuration of plagioclase phenocrysts： they appear euhedral in the former and flattened in the latter. Flow foliations tend to parallel the contact between the pluton and the wall rocks, and they are non-developed, underdeveloped, and developed towards the center. On this point, the ballooning model can best explain the emplacement of the pluton. Owing to the dramatic decrease in magma supply at the final pulse, the strain intensity tends to increase towards the center, as is different from the strain distribution observed in classic ballooning plutons. Tectonic gneissosity is much more locally distributed, but strikes towards northeast in general, indicating that it should have formed in the subsolidus or solidus state during the NW-SE regional compression.