Anatomy of the Cretaceous Hobenzan pluton, SW Japan: Internal structure of a small zoned pluton, and its genesis

作者：Imaoka Teruyoshi^*; Nakashima Kazuo; Kamei Atsushi; Hayasaka Yasutaka; Ogita Yasuo; Ikawa Toshiyuki; Itaya Tetsumaru; Takahashi Yoshio; Kagami Hiroo

来源：Lithos, 2014, 208: 81-103.

DOI：10.1016/j.lithos2014.09.002

• Zoned pluton
• Cretaceous
• Fractional crystallization
• Crystal-melt separation
• Sr-Nd isotopes
• SW Japan

Field, petrographic, geochemical, and K-Ar and U-Pb age data were used to elucidate the internal structure of the Cretaceous Hobenzan pluton, SW Japan, and the processes which generated that structure. The pluton is elongated E-W with dimensions of about 6.5 x 2.0 km (13 km(2)), and was emplaced at -95 Ma as a pluton in accretionary complexes. The pluton contains an early tonalite, but most of the body consists of later granitoids that show a continuous differentiation series from biotite-hornblende granodiorite (GD) to hornblende-biotite granite (HBG) and biotite granite (BG). The contacts between the GD and HBG are gradational. The pluton provides an exceptional cross-sectional view of a simple cooling magma body. The GD shows no vertical variations in modal and chemical compositions, whereas the HBG displays differentiation from the lowermost exposure to the top of the pluton. Initial Sr isotope ratios (Srl) in the HBG increase from the lower part to the top of the pluton. The granitoids show continuous compositional variations from 65 to 79 wt% SiO2 (anhydrous basis), and magmatic differentiation was dominantly controlled by crystal fractionation of hornblende and plagioclase. Field, elemental and Sr-Nd isotope data are consistent with limited operation of assimilation with pelitic rocks and fractional crystallization (AFC), in which assimilation increased with higher degrees of differentiation. The Hobenzan pluton retains a history of granitoid magma evolution in a subvolcanic magma reservoir. The GD formed as a rigid sponge, and melt fraction increases inwards from the walls, forming the HBG mush by fractional crystallization, coupled with small degrees of assimilation of adjacent schists. A more evolved and enriched lowdensity melt segregated from the mushy cumulate of the HBG by incomplete crystal-melt separation, and moved upwards with the assistance of gas-driven filter pressing, as indicated by the presence of miarolitic cavities, thus forming the BG at the roof of the pluton.

• 出版日期2014-11