Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.

长英质岩石数量较少，但在西南印度洋脊的国际大洋发现计划U1473A钻孔中普遍存在。脉体中高钛棕色角闪石、斜长石和锆石的微量元素丰度，以及所研究的长英质岩石中蠕状结构的存在，都支持其源自高度演化的熔体的结晶成因，这可能是受分离结晶作用控制。根据地球化学标准以及长英质岩石中矿物组合的结构及其与寄主辉长岩的关系，它们可分为三种类型：（1）当长英质熔体侵入寄主辉长岩的裂隙时，形成边界清晰的长英质岩石，导致熔体与围岩矿物之间的相互作用极小。（2）交代型长英质岩石，其特征是寄主辉长岩中的矿物发生假像交代。这种脉体类型是由与长英质熔体平衡的矿物对寄主矿物的交代作用引起的。（3）边界扩散的长英质岩石是由长英质熔体渗入正在凝固的辉长岩体或间隙熔体结晶形成的。长英质熔体的渗入模式可能受冷却的寄主辉长岩的温度条件控制。