Generation of porosity network in plagioclase induced by multistage infiltration of reactive fluids into metamorphic rocks around quartz diorite intrusion at the middle crust

Abstract:

In volcanic zone, geophysical observations infer the existence of deep-seated fluids above the plutonic intrusions at the middle crust. However, the fluid compositions and the mechanism of fluid migration at high temperature crust are still poorly understood. In this study, we investigated the plagioclase (Pl) alteration in mafic schists (clinopyroxene (Cpx) schist, hornblende (Hbl) schist, actinolite (Act) schist) at the contact with quartz diorite intrusion in Kinkasan Island, NE Japan. The quartz diorite is composed of Hbl and Pl which show the P-T conditions of 670–760 ˚C and 0.3–0.45 GPa. It contains abundant pegmatitic veins that develop cut the schistosity of metamorphic rocks. In these rocks, at least two stages of fluid-mediated alteration processes occurred. First stage (Stage 1) is characterized by the reaction zone around pegmatite veins which cut Cpx schists. This zone was marked by the replacement of Pl (anorthite (An80-90)) and Cpx with Na-rich Pl (An40-60) and Hbl along the grain boundaries, respectively. The second stage (Stage 2) is defined as K-feldspar (Kfs) veins and replacement of Pl by Kfs (An0Ab1Or99) and albite (An4Ab94Or2). Assuming the same pressure of intrusion, temperatures of alteration are estimated to be 690-730 ˚C for Stage 1, and 400-570 ˚C for Stage 2, respectively. Mass balance analyses of Pl (An35-60) replacement show the difference of element transfer between the stages which reveal fluid composition changes from Na-rich (Stage 1) to K-rich (Stage 2) fluids during cooling history. Interestingly, significant nano-micro scale pores were generated during K-rich fluids alteration, which increases bulk rock porosity by 1.34±0.14%. These pores are isolated and occur along the replacement front, suggesting that pores migrate in progress of replacement. The reaction front is likely defined by the trans-grain fracture than the primary plagioclase grain boundary. Our findings suggest that an infiltration of K-rich fluids produces pervasive fluid pathways through replacement processes. As plagioclase is dominant mineral in middle crust, these reaction-induced porosity networks could act as dominant way of fluid transports. The results of this study indicate that porosities and fluid migration at middle crusts are potentially controlled by the compositions of the reactive fluids.