Occurrence of pyrope-rich garnets and large chemical variations of garnet porphyloblasts in the whiteschist from the Kulet area of the Kokchetav Massif

Monday, 15 December 2014
Tomohiro Takebayashi, Kunihiko Sakamaki and Yoshihide Ogasawara, Waseda University, Tokyo, Japan
Garnet porphyroblasts having extremely high pyrope-components (up to 68 mol%) were found in whiteschist from the Kulet area of the Kokchetav Massif. The pyrope components are much higher than those previously reported (e.g., Prp: up to 12.37 mol% by Parkinson, 2000). Two kinds of garnet porphyloblasts were used in this study; group (A): just porphyloblasts without matrices by the excavation along trenches and weathering of the whiteschist, and group (B): porphyroblasts included in the matrix of whiteschist (sample no. ZW42). 42 grains of garnet porphyloblasts were analyzed with a standard-less quantitative EDS system. Garnet porphyroblasts of both (A) and (B) groups show clear chemical zonations and have higher pyrope components at the rim. Large chemical variations were recognized at grain by grain.

Group (A) garnets, which are probably originated from the similar whiteschist as sample no. ZW42, are subhedral to euhedral crystals (5-35 mm across) and show strong chemical zonations of Mg and Fe from the core (Prp: 15-23, Alm: 64-70 mol%) to the rim (Prp: 39-68, Alm: 30-61 mol%). Large variation of pyrope components at the rims were identified among the grains analyzed. Group (A) garnets include chloritoid, Rt, and corundum at the core, and Rt, Chl, Ky, Zr, and monazite at the rim.

In whiteschist (sample no. ZW42), the matrix shows lepidobalstic texture and consists of Phe, Bt, Qtz, and Ky; similar to that by Parkinson (2000). The garnet porphyroblasts (group B) measure 4-15 mm across and show chemical zonations of Mg and Fe from the core (Prp: 21-25, Alm: 71-72 mol%) to the rim (Prp: 27-34, Alm: 66-72 mol%). These garnets include chloritoid, Qtz, Rt, Chl and Ap at the core, and Rt, Zrn, and monazite at the rim.

The examined Kulet garnets show large chemical variations, particularly about the pyrope components, and the difference in inclusion mineralogy, although the host rocks of both group garnets might be the similar whiteschist. The big question arising here is why such large chemical variations of garnet porphyloblasts were observed in the whiteschist with similar occurrences. One possible explanation is that the strong retrograde alteration in the matrix might have erased the difference in the bulk rock chemistries of the protoliths. Only porphyroblasts might have held the information about the protoliths.