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摘要
High-precision U-Pb isotope dilution - thermal ionization mass spectrometry (ID-TIMS) geochronology on chemically abraded zircon grains from a noritic gabbro of the Ni-bearing Suwar mafic-ultramafic layered complex, northwestern Yemen, gives a mean (206)Pb/(238)U age of 638.46 +/- 0.73 Ma (2 sigma; MSWD = 1.4). At Wadi Qutabah, similar to 30 km to the north, a similar mafic sample has an identical age of 638.58 +/- 0.51 Ma (2 sigma; MSWD = 0.32), which supports the possibility of there being a single, large intrusive complex with an estimated areal extent of similar to 250 km(2). This is supported by geochemical data of samples from each locality, which are postkinematic, gabbroic rocks that contain variable amounts of cumulus olivine, plagioclase, orthopyroxene, and ilmenite with intercumulus augite, hornblende, and Ni-sulphides. Straight rare Earth element (REE) patterns, Ba/La similar to 30, Rb/Ba similar to 0.04, and negative primitive-mantle-normalized P anomalies resemble EM1 (Enriched Mantle 1) of oceanic island basalts and Archean subcontinental mantle lithosphere. The mineralogy and magmatic/tectonic discrimination diagrams suggest within-plate continental tholeiitic and noritic cumulates typical of a rift setting for both intrusions. The complex intrudes retrograded, amphibolite-facies paragneiss of the Pan-African Afif lithotectonic terrane, and is generally undeformed and unaltered, and, therefore, unaffected by the Pan-African orogeny. Emplacement of the 639 Ma complex occurred during a tensional tectonic regime on the Arabian Peninsula and marks the time of proto-Iapetan rifting. The estimated size of the intrusion, its noritic composition, and Archean subcontinental lithospheric mantle signature and its position in thin Proterozoic lithosphere abutting Archean cratonic rocks, give it the key characteristics of known, mostly Proterozoic, intrusions that host world-class Ni-Cu-Co ore deposits.
- 出版日期2011-2
