We investigated the fluid flow and elemental transport from a granitic body to the middle crust by determining the trace element compositions of garnet in pegmatites related to a quartz diorite intrusion and metamorphic rocks on Kinkasan Island, northeast Japan. Garnet in the pegmatites and biotite schists is characterized by spessartine– (Sps–) almandine- (Alm-) rich compositions of Sps14–69Alm22–70Prp2–14Grs1–13 and Sps16–30Alm54–66Prp9–16Grs3–6, respectively. A garnetite pod in the metamorphic unit has grossular- (Grs-) rich compositions (Sps1–4Alm8–11Prp0.1–0.4Grs80–87Adr3–4). The peak temperature ( T ) and pressure ( P ) conditions of the biotite schist during contact metamorphism were 600–650°C and 0.27–0.41 GPa, respectively. The primary fluid inclusions in quartz crystals within the pegmatites hosted by the quartz diorite and hosted by the metamorphic rocks have a wide range of homogenization temperatures (200–380°C). These correspond to the trapping temperature of 500–700°C, assuming a salinity of 4 wt.% NaClequivalent at pressure of the crystallization of the quartz diorite. Chondrite-normalized rare earth element (REE) patterns of garnets in the pegmatites in the quartz diorite and metamorphic unit are generally characterized by enrichment of heavy REEs and negative Eu anomalies with the REE contents in the schists which are systematically lower than in the pegmatites. However, garnet in the biotite schists close to the pegmatites has similar REE contents to garnet in the adjacent pegmatites. These geochemical features suggest that garnet in the biotite schists grew in response to fluid infiltration from the pegmatites. Besides, the Grs-rich garnet in the garnetite pod and its host quartz schist have flat heavy REE patterns and no Eu anomalies, which probably reflect a metasomatic process related to plagioclase replacement that produced Ca-Al-rich fluids. Our results suggest that the infiltration of pegmatitic fluids enhances elemental transport and metamorphic reactions in the middle crust.
<p>Replacement of feldspars occurs ubiquitously during fluid-rock interaction in crusts, and the formation of micro- to nano- pores along with the replacement potentially provides significant impacts on hydrological properties within the crust (e.g. Pl&#252;mper et al., 2017; Yuguchi et al., 2019). In this contribution, we report the novel texture of the plagioclase replacement by K-feldspar and albite and showed the conditions of such replacement. The mafic schists near the pegmatitic quartz diorite within the Kinkasan Island, NE Japan show extensive feldspar alteration at various stages, involving Na-rich and K-rich fluids, respectively. Interestingly, during the later K-rich fluid infiltration at 400-570 &#730;C at 0.3&#8211;0.45 GPa, plagioclase (An35-60) was replaced by K-feldspar (An0Ab1Or99) and albite (An4Ab94Or2) intergrowth, meaning that simultaneous K-feldspathization and albitization, and nano- to microscale pore network developed preferentially along with albite, resulting in an increase of the bulk rock porosity up to 1.34&#177;0.14%.</p><p>To understand the relationship between K-feldspar and albite formations within the same plagioclase grain, we conducted the hydrothermal experiments on the feldspar replacement by using different pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) and fluid compositions (2M KCl and/or NaCl aqueous solutions) for 4-8 days. AIn all runs, the replacement processes of feldspars developed the distinct reaction front and pores formation close to the reaction front with porosity up to ~7%. In the experiments with KCl solution, the reaction front migrated twice faster than those with the mixture of KCl and NaCl. The most intense replacement occurred in the run of Labradorite-KCl solution, where large cavities were formed in the center of the labradorite grain with developing albite exsolution, and homogenous rim of K-feldspar precipitation. Such occurrences are similar to the replacement texture observed in the mafic schist within the Kinkasan Island and suggest the preferential removal of Ca and the fixed Na during K-feldspar formation. Our experimental results indicate the primary controls of the fluid composition on the replacement texture, pore formation, and the reaction rate.</p><p>Keywords feldspar replacement, micropores, fluid transport, hydrothermal experiment, Kinkasan</p>
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