科研之友
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摘要
Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern Oman. The diatreme consists of heterogeneous deposits dominated by 'diatreme facies' pyroclastic rocks. These include aillikite and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain nappes. Both rock types are dominated by carbonate, altered olivine, Ti-Al-phlogopite and Cr-Al-spinel and contain varying amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized by low SiO(2) (11-24 wt%), MgO (9.5-12.4 wt%) and K(2)O (< 0.3 wt%), but high CaO (18-22 wt%), Al(2)O(3) (4.75-7.04 wt%), Fe(2)O(3tot) (8.7-13.8 wt%) and loss-on-ignition (24-30 wt%). Higher CaO, Fe(2)O(3total), Al(2)O(3), MnO, TiO(2), P(2)O(5) and lower SiO(2) and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite, 66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite, carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep slopes with (La/Sm) (n) of 6-10 and relative depletion in heavy rare earth elements (Lu = 3-10 chondrite). Initial (87)Sr/(86)Sr ratios vary from 0.70409 to 0.70787, whereas initial (143)Nd/(144)Nd ratios vary between 0.512603 and 0.512716 (epsilon Nd (i) between 2.8 and 3.6). (206)Pb/(204)Pb (i) ratios vary between 18.4 and 18.76, (207)Pb/(204)Pb (i) ratios vary between 15.34 and 15.63, whereas (208)Pb/(204)Pb (i) varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 +/- A 1 Ma (95% confidence, MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140-160 Ma. Geochemical and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean.
- 出版日期2011-1
