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Sensitive high-resolution ion microprobe U–Pb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000–300 Ma zircon. Older, 1000–570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized: 相似文献
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Transformation Trillings in Cordierite 总被引：1，自引：0，他引：1
Cordierite sector trillings, which are found well developedin some A1-rich volcanic rocks and pelitic buchites, are thoughtto have formed initially as hexagonal high cordierite crystals.Subsequent ordering of the (Al, Si) configuration in the crystalstructure would have produced the present orthorhombic modificationand simultaneously the sector trilling, preserving in the processthe hexagonal habit of form. The predominance of the regularsixfold sector pattern in the composition plane configurationof the resulting trilling is best explained by assuming thatits formation is kinetically more advantageous than that ofother configurations because it mimics the hexagonal trend setby the initial crystal. The three orthorhombic orientations in the cordierite trillingshave parallel c-axes and each can be transformed into eitherof the other two by operation of a threefold axis (rotationthrough 120°) parallel to the common c-axis and coincidentwith the main axis of the original hexagonal form. The compositionplanes in the sector trilling pattern are shown to be essentiallyirregular in the present orthorhombic form; they do not belongto {110} or {130}. The traditional explanation that the trillings are multipletwins with repeated twin and composition planes of {110} or{130} is rejected since elementary geometrical considerationsshow that this model is only feasible if (110) (110) of orthorhombiccordierite equals 120°. X-ray work shows conclusively thatthis is not the case. In fact (110) (110) = 120° woulddefine a hexagonal symmetry. 相似文献
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Ion microprobe U–Pb dating of a bimodal zircon populationfrom a graphitic schist from the basal section of the AlpineAlpujárride nappe complex in the Betic Cordilleras, southernSpain, has yielded different ages for zircons of contrastingmorphology and composition. Approximately half the grains arerounded and abraded, and have ages ranging from 相似文献
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Almandine–graphite-bearing biotite–cordierite dacitefrom Cerro del Hoyazo, southern Spain, represents erupted magmaproduced by Late Alpine anatexis of high-grade pelitic/quartzo-feldspathicparagneisses. U–Th–Pb ion microprobe analysis ofinherited/detrital zircon cores from the dacite reveals fiveprincipal age groups, 2·8–2·5 Ga, 2·1–1·9Ga, 1·1–0·9 Ga, 650–550 Ma and 350–320Ma, indicating a Gondwana domain provenance. Some of the coresare surrounded by a thin euhedral overgrowth which has the sameage as a suite of sharply euhedral new-grown zircons in thedacite, 6·33 ± 0·15 (t 相似文献
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