Discussion on SHRIMP U–Pb zircon dating of the exhumation of the Lizard Peridotite and its emplacement over crustal rocks: constraints for tectonic models
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In this recent contribution, Nutman et al. (2001) provide the first SHRIMP U–Pb zircon age data for the Lizard Peridotite and its immediate envelope, and demonstrate the usefulness of ion-microprobe geochronology in the unravelling of the complex thermo-tectonic history inherent in the emplacement of such assemblages of plausibly oceanic rocks onto continental strata. Building on the research of Cook et al. (2000), the authors distinguish clearly between the Lizard Peridotite proper , a mass of c. 15 kbar mantle tectonite, and the contiguous Crousa Downs complex of gabbro, troctolite and sheeted mafic dykes, emplaced into the peridotite following its exhumation and cooling. Thus, although exhibiting many lithologies and structures characteristic of ophiolites, the “oceanic” part of the Lizard Complex is clearly at least bipartite. The studies of Cook et al. (2000) and Nutman et al. (2001) rationalize and clarify several problematic relationships in this extensively studied, but still ambiguous suite.
We understand that the editorial handling of Nutman et al. ’s paper overlapped with the publication in this journal of our detailed account of the petrogenesis of the Kennack Gneiss, which incorporates an evaluation of the tectonic context of this controversial unit on the basis of an IDTIMS single-zircon date (Sandeman et al. 2000). We consider, none the less, that several aspects of the new contribution require comment. These include Nutman et al. ’s interpretations of our earlier documentation of the petrology and age relationships of the Kennack Gneiss (Sandeman et al. 1995), the Man of War Gneiss (Sandeman et al. 1997) and the Porthkerris Plagiogranite (Clark et al. 1998 b ), as well as our overview of the emplacement history of the Lizard Complex (Clark et al. 1998 a ), an extended abstract cited but not referenced by Nutman et al. We employ herein the revised Palaeozoic time-scale of McKerrow …Keywords:
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Wulandele plulon is a large-scale complex massif generated by twice magmatism.The complex massif is made up of biotite-granite and biotite-monzogranite,aged in 140±2 Ma and 161±1 Ma,respectively.The biotite-granite reflects highly fractionated characteristics geochemically.While the biotite-monzogranite implies low Sr and high Yb in geochemical characteristics.Geochemical data suggest that the biotite-monzogranite are A-type granites,which tectonic setting was are mainly dominated by the extensional system.
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The Monviso metamorphic ophiolite, one of the best preserved relics of oceanic crust in the Western Alps, was formed during the opening of the Mesozoic Western Alpine Tethys and underwent metamorphism to eclogitic conditions during Alpine subduction. The Monviso ophiolite encompasses the whole lithological spectrum of the Piedmont-Liguria ophiolite rocks, with a basal unit of serpentinized peridotite in tectonic contact with the overlying metagabbros, eclogites and pillowed metabasalts. Slivers of serpentinized peridotite hosting banded eclogites and metagabbros divide these units from the overlying Forciolline Unit. The latter (formerly called Costa Ticino Series) is an overturned sequence of gabbros with pods of cumulate troctolite and lenses of serpentinized peridotite, overlain by massive and pillow metabasalts. A unit of massive metabasalts tops the tectonic stack. A body of jadeite-quartz bearing metaplagiogranite has been recently found in the Basal Serpentinite Unit near Verne, northwest of Sampeyre, Val Varaita. Zircon crystals recovered from the Verne metaplagiogranite have large domains with typical magmatic zoning with broad oscillatory bands. They have Th/U ratios in the range 0.3-0.7, as commonly observed in magmatic zircon. In situ ion microprobe (SHRIMP) U-Pb dating of the magmatic domains yielded a mean age of 152±2 Ma, which is interpreted as the crystallization age of the Monviso plagiogranite. Unzoned domains that crosscut magmatic zircon yielded younger, apparent ages which are most likely due to Pb loss during Alpine metamorphism. In conjunction with previous works on ophiolites from the Western Alps, Northern Apennines, and Alpine Corsica, the new data from Monviso suggest that the plutonic activity in the Piedmont-Liguria domain of the western Tethys may have lasted only 15 to 20 Ma, between ca. 170 and ca. 150 Ma. As shown by Radiolarian biostratigraphy, this is approximately the same time span encompassed by the extrusion of tholeiite basalts which cap both the gabbro plutons and their peridotite country rocks. The new data indicate that the plutonic activity recorded at Monviso was coeval with basalt extrusion and deep-sea sediment deposition in some Liguriantype ophiolite bodies of the Cottian Alps. This suggests that the oceanic crust preserved in the Monviso ophiolite may have formed later, and in a more central position of the basin, than the oceanic crust preserved in such Ligurian-type ophiolite bodies.
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The Dulong Sn-Zn deposit is one of the largest cassiterite-sulfide deposits in China.There are different opinions regarding the genesis of this deposit,partly due to the lack of reliable geochronological data.We report here results of the first attempt to determine the magmatic and mineralization ages of this deposit and related late-Yanshanian granites through U-Pb dating of cassiterite and zircon.Four cassiterite samples from the Manjiazhai section yields a means~(206)Pb/(238)U age of 79.8±3.2Ma(MSWD=3.16), and a~(238)U/~(204)Pb-~(206)Pb/~(204)Pb isochronal age of 82.0±9.6Ma(MSWD=4.81)using the TIMS U-Pb method.Zircon from a concealed granite at the Manjiazhai section yielded a means~(206)Pb/~(238)U age of 92.9±1.9Ma(N=10,MSWD=0.71),and zircon derived from a granitie-porphyry outcrop at the Tongjie section yielded a means~(206)Pb/~(238)U age of 86.9±1.4Ma(N=9,MSWD=3.70),both obtained using the SHRIMP U-Pb method.These ages suggest that the tin(copper)mineralization in the Dulong Sn-Zn deposit was mainly related to the late Cretaceous magmatic hydrothermal events.The geochronological data of the Gejiu(Sn)and the Bainiuchang (Ag)super-large deposits and related Yanshanian granites indicate that there was a large-scale granitic magmatism and mineralization event during the Cretaceous in southeastern Yunnan,which might have been related to the lithospheric extension of the South China Block in late Mesozoic.
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The Jijal ultramafic–mafic complex in Pakistan probably preserves the most complete fragments of the petrological Moho. However, a few studies argue for multiple origins (including petrogenetic speculations and tectonic reconstructions) for different lithologies. One of the main reasons for this dispute is the lack of direct age information of the ultramafic rocks. Zircon grains, despite generally being exotic in ultramafic rocks, can provide significant insights into the petrogenetic process of the host ultramafic rocks. This study reports the first zircon U–Pb age and Lu–Hf and trace element data for zircon grains separated from chromitite lenses within the peridotite, which is commonly considered the lowermost part of the Jijal complex. These zircon grains yield concordant 206Pb/238U ages of ~182 ± 3 Ma, which is much older than the late Early Cretaceous age (<120 Ma) of the Jijal complex, and lying above it, the other complexes of the Kohistan paleo-arc. Furthermore, these Jurassic zircon grains present radiogenic εHf(t) values (+9.7 to +6.0) which are obviously lower than the values for the Cretaceous zircon grains of the Kohistan arc. From integrated analysis of the zircon trace element signatures (e.g., high Th, U, Th/U, and U/Yb ratios) and regional geology, we speculate that these zircon grains came from a ‘missing’ Early Jurassic arc akin to the Gangdese belt to the east, and entered the mantle by oceanic subduction processes. Although these Jurassic zircon grains cannot actually constrain the formation age of the chromitite as well as the peridotite, it reminds us that some cryptic pre-Cretaceous complexes and geodynamic processes were incorporated in building the oceanic crust of the Jijal intra-oceanic arc, or the mantle section (at least part of it) should probably belong to the Indus ophiolite mélange. Further research, particularly chronological studies on mantle (or ultramafic) rocks, as well as detailed geological mapping, should be carried out in the future for solving this issue.
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