Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints
Resource
Lithos 70,243-267
Journal
Lithos 70
Pages
243-267
Date Issued
2003
Date
2003
Author(s)
DOI
246246/2006111501254510
Abstract
The Maowu eclogite –pyroxenite body is a small (250 50 m) layered intrusion that occurs in the ultra-high-pressure (UHP)
metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a
crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the
collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic
investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic
behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial
87
Sr/
86
Sr ratios
(0.707 –0.708) and inhomogeneous negative eNd (T) values of 3 to 10 at 500 Ma (probable protolith age). Post-magmatic
and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock
(WR) Sm–Nd isochronal relationship, low d
18
O values and an extreme enrichment of Th and REE in a clinopyroxenite.
Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial
87
Sr/
86
Sr ratios unsupported by in situ
Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during
UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are
not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as
monazite and apatite, is required to account for the REE budget.
Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221 F5 Ma and e(T)=5.4 for an
eclogite, 231 F16 Ma and e(T)=6.2 for a garnet websterite, and 236 F19 Ma and e(T)=6.9 for a garnet clinopyroxenite.
The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned
above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–
Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar
in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber
processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the
upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths,
UHP metamorphism and return to the crustal level.
Subjects
Dabieshan
Maowu
UHP metamorphism
Eclogite
Pyroxenite
Sm–Nd isotopes
Age
Type
journal article
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