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Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy) 总被引：3，自引：0，他引：3
Garnet clinopyroxenite and garnet websterite layers occur locallywithin mantle peridotite bodies from the External Liguride Jurassicophiolites (Northern Apennines, Italy). These ophiolites werederived from an ocean–continent transition similar tothe present-day western Iberian margin. The garnet clinopyroxenitesare mafic rocks with a primary mineral assemblage of pyrope-richgarnet + sodic Al-augite (Na2O 2·5 wt %, Al2O3 12·5wt %), with accessory graphite, Fe–Ni sulphides and rutile.Decompression caused Na-rich plagioclase (An50–45) exsolutionin clinopyroxene porphyroclasts and extensive development ofsymplectites composed of secondary orthopyroxene + plagioclase(An85–72) + Al-spinel ± clinopyroxene ±ilmenite at the interface between garnet and primary clinopyroxene.Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under syn-kinematicconditions, at the expense of two-pyroxenes + Al-spinel. Mg-richgarnet has been also found in the websterite layers, which arecommonly characterized by the occurrence of symplectites madeof orthopyroxene + Al-spinel ± clinopyroxene. The enclosingperidotites are Ti-amphibole-bearing lherzolites with a fertilegeochemical signature and a widespread plagioclase-facies myloniticfoliation, which preserve in places a spinel tectonite fabric.Lu–Hf and Sm–Nd mineral isochrons (220 ±13 Ma and 186.0 ± 1·8 Ma, respectively) have beenobtained from a garnet clinopyroxenite layer and interpretedas cooling ages. Geothermobarometric estimates for the high-pressureequilibration have yielded T 1100°C and P 2·8 GPa.The early decompression was associated with moderate cooling,corresponding to T 950°, and development of a spinel tectonitefabric in the lherzolites. Further decompression associatedwith plagioclase–olivine growth in both peridotites andpyroxenites was nearly isothermal. The shallow evolution occurredunder a brittle regime and led to the superposition of hornblendeto serpentine veining stages. The garnet pyroxenite-bearingmantle from the External Liguride ophiolites represents a raretectonic sampling of deep levels of subcontinental lithosphereexhumed in an oceanic setting. The exhumation was probably accomplishedthrough a two-step process that started during Late Palaeozoiccontinental extension. The low-pressure portion of the exhumationpath, probably including also the plagioclase mylonitic shearzones, was related to the Mesozoic (Triassic to Jurassic) riftingthat led to continental break-up. In Jurassic times, the studiedmantle sequence became involved in an extensional detachmentprocess that resulted in sea-floor denudation. KEY WORDS: garnet pyroxenite; ophiolite; non-volcanic margin; mantle exhumation; Sm–Nd and Lu–Hf geochronology 相似文献
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The mafic granulites from the Northern Apennines commonly containsignificant amounts of either olivine or Fe–Ti-oxide phases.On the basis of mineralogy and whole-rock major and trace elementcompositions, their protoliths are recognized as cumulus rocksderived from variably evolved tholeiitic liquids. Clinopyroxenesfrom the olivine-bearing rocks show peculiar trace element compositions(e.g. low amounts of Cr, Zr and lanthanides, coupled with relativelyhigh Sr concentrations) that record a process of granulite-faciestrace element redistribution controlled by a reaction betweenolivine and plagioclase. The trace element compositions of clinopyroxenesfrom the Fe–Ti-oxide-bearing rocks point to igneous geochemicaltrends that argue for a common igneous parentage. The 相似文献
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