Petrogenesis of the garnet peridotite and garnet-free peridotite of the Zhimafang ultramafic body in the Sulu ultrahigh-pressure metamorphic belt, eastern China

作者：Yang J S^*; Zhang R Y; Li T F; Zhang Zh M; Liou J G

来源：Journal of Metamorphic Geology, 2007, 25(2): 187-206.

DOI：10.1111/j.1525-1314.2006.00688.x

• CCSD-PP1
• garnet peridotite
• mantle
• Sulu
• UHP

The CCSD-PP1 drillhole penetrated a 110-m-thick sequence of the Zhimafang ultramafic body in the Sulu ultrahigh-pressure (UHP) metamorphic belt, east China. The sequence consists of interlayered garnet-bearing (Grt) and garnet-free (GF) peridotite. Eleven layers of Grt-peridotite, ranging from 1.2 to 9.5 m in thickness, have an aggregate thickness of 54.49 m, whereas eight layers of GF-peridotite, ranging from 2.2 to 14.2 m in thickness, have a total thickness of 57.53 m. The boundaries between the two rock types are gradational. The Grt-peridotites have slightly higher contents of Al(2)O(3), CaO and SiO(2), and lower Mg(#)s (0.90-0.92) than the GF-peridotites (Mg(#)s 0.91-0.93). Both contain low TiO(2) (< 0.05 wt%) and have higher modal abundances of enstatite (average 10 vol.%) than diopside (1-5 vol.%), typical of depleted-type upper mantle. The diopside in these rocks has high and relatively uniform Mg(#) members (0.93-0.95), but highly variable Al(2)O(3) (0.2-2.3 wt%), Na(2)O (0.5-2.5 wt%) and Cr(2)O(3) (0.38-2.09 wt%). Enstatite (En(92-93)) contains very low Al(2)O(3) (0-0.3 wt%). Both porphyroblastic and equigranular garnet are present. The equigranular varieties are zoned, from core to rim in Cr(2)O(3) (3.4-4.2 wt%), MgO (18.4-17.5 wt%) and Al(2)O(3) (21.1-20.1 wt%). Titania is very low in all the garnet, mostly < 0.05 wt%. Chromite or chromium (Cr)-spinel occur both in the Grt- and GF-peridotite, and are characterized by high contents of Cr(2)O(3) (49-58 wt%) and FeO (24-43 wt%), similar to that in iron-rich Alpine-type peridotites. Based on the bulk-rock MgO-FeO compositions, the Zhimafang Grt-peridotite probably underwent 20-30% partial melting, whereas the GF-peridotite may have undergone as much as 35-40% partial melting, suggesting that the two rock types owe their differences to different degrees of partial melting rather than to pressure differences during metamorphism.

• 出版日期2007-2
• 单位中国地质科学院