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Petrogenesis of the garnet peridotite and garnet‐free peridotite of the Zhimafang ultramafic body in the Sulu ultrahigh‐pressure metamorphic belt, eastern China
Author(s) -
YANG J. S,
ZHANG R. Y.,
LI T. F.,
ZHANG Zh. M.,
LIOU J. G.
Publication year - 2007
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2006.00688.x
Subject(s) - peridotite , enstatite , geology , geochemistry , ultramafic rock , diopside , metamorphic rock , mantle (geology) , olivine , mineralogy , petrology , chondrite , meteorite , physics , astronomy
Abstract 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.