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491.
Lewis D. Ashwal Michael A. Hamilton Vincent P. I. Morel Roger A. Rambeloson 《Contributions to Mineralogy and Petrology》1998,133(4):389-401
Four massif-type anorthosite bodies 25–100 km2 in area occur within high-pressure granulite facies supracrustal gneisses in southwestern Madagascar. Two of these bodies
(Ankafotia and Saririaky) appear to have been pulled apart by 40 km in a ductile shear zone, but structural features such
as sub-vertical stretching lineations indicate an origin by intense west-directed flattening and pure shear. Country rocks
(Graphite Series) include abundant graphite schist (some with >60% graphite), marble, quartzite, and minor amphibolite and
leucogneiss. Comagmatic granitoids (e.g. charnockites) are conspicuously absent. The anorthosite bodies are dominated by coarse
grained anorthosites and leuconorites (feldspars typically 3–5 cm, up to 1 m); minor norites and oxide-rich ferrogabbros occur
near the margins, but ultramafic rocks are absent. Typical mineralogy of the anorthositic rocks is: plagioclase (An41–54) + orthopyroxene (En38–66) ± augite (Mg♯ = 32–68) ± ilmenite ± magnetite ± apatite. High-alumina (to 6.1 wt% Al2O3) orthopyroxene megacrysts are widespread; most have exsolutions of calcic plagioclase (An72–85) but some contain garnet lamellae. Metamorphism has produced abundant recrystallization and sporadic coronitic garnet (Mg
♯=12–36) + clinopyroxene assemblages. Rb-Sr isotopic analyses of whole-rocks and minerals reveal no meaningful age relationships.
The age of late Neoproterozoic metamorphism is best constrained at 559 ± 50 Ma by a 6-point Sm-Nd mineral isochron (whole
rock, plag, pyx, ilm, apat, gar) from a Saririaky oxide-rich gabbro. The igneous crystallization age of the anorthosites is
estimated at 660 ± 60 Ma by a 19-point combined whole-rock and mineral Sm-Nd isochron for samples from both the Ankafotia
and Saririaky bodies. Initial isotopic ratios calculated at 0.66 Ga among 13 whole rocks are: Nd=+2.6 to +5.2 (mean=+3.7) and ISr=0.70328–0.70407 (mean=0.70347), indicating derivation of the Malagasy anorthosites from a depleted mantle source, and little,
if any, contamination with Archean crustal material. One anorthosite sample with Nd=−1.4 and ISr=0.70344 (calculated at 0.66 Ga) probably reflects the effects of assimilation of Early to Middle Proterozoic crustal basement,
but typical surrounding graphite schist (Nd=+0.3, ISr=0.70636, both at 0.66 Ga; TDM= 1131 Ma) represents only a minor potential contaminant for the anorthosite bodies. TDM model ages of the Malagasy anorthosites (797–1280 Ma; mean of 14 samples=949 Ma), as those of most other massif-type anorthosites,
are older than the true crystallization age, because of crustal contamination effects. Our isotopic data, together with recent
U-Pb data from the anorthosites and surrounding country rocks, are consistent with emplacement of the Malagasy anorthosite
bodies at or before the start of a protracted, high-grade metamorphic event or series of events between about 630 and 550 Ma.
This period coincides with the collision between, and amalgamation of, East and West Gondwana.
Received: 19 December 1997 / Accepted: 12 June 1998 相似文献