Home About us Contact
|
Home About us Contact | ||
|
|
||
Upper Amphibolite Facies (upper + amphibolite_facy)
Selected AbstractsAmphibolite and blueschist,greenschist facies metamorphism, Blue Mountain inlier, eastern JamaicaGEOLOGICAL JOURNAL, Issue 5 2008Richard N. Abbott Jr Abstract Cretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault-bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS),greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P,T paths. The paths are independent, but consistent with different parts of the type-Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P,T conditions support a two-stage P,T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P ,7.5,kbars, T ,600°C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P,=,2.6,3.0,kbars, T,=,219,237°C for a(H2O),=,0.8,1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P,T path of the sort that produced the Westphalia Schist. Reset K,Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5,Ma (pre-Campanian). Uplift may have commenced with an Albian,Aptian (,112,Ma) orogenic event. Copyright © 2008 John Wiley & Sons, Ltd. [source] Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, JapanISLAND ARC, Issue 1 2000Daniel K. Asiedu Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high-grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote-amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle-derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted. [source] Constraints on the early metamorphic evolution of Broken Hill, Australia, from in situ U-Pb dating and REE geochemistry of monaziteJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2009C. R. M. McFARLANE Abstract The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c.1630 Ma and c.1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c.1657 Ma. [source] Timing relationships between pegmatite emplacement, metamorphism and deformation during the intra-plate Alice Springs Orogeny, central AustraliaJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2008I. S. BUICK Abstract In the Harts Range (central Australia), the upper amphibolite facies to lower granulite facies, c. 480,460 Ma Harts Range Metamorphic Complex (HRMC), and the upper amphibolite facies, c. 340,320 Ma Entia Gneiss Complex are cut by numerous, generally peraluminous pegmatites and their deformed equivalents. The pegmatites have previously been interpreted as locally derived partial melts. However, SHRIMP U,Pb monazite and zircon dating of 29 pegmatites or their deformed equivalents, predominantly from the HRMC, reveal that they were emplaced episodically throughout almost the entire duration of the polyphase, c. 450,300 Ma intra-plate Alice Springs Orogeny. Episodes of pegmatite intrusion correlate with the age of major Alice Springs-age structures and with deposition of syn-orogenic sedimentary rocks in the adjacent Centralian Superbasin. Similar Alice Springs ages have not been obtained from anatectic country rocks in the HRMC, suggesting that the pegmatites were not locally derived. Instead, they are interpreted as highly fractionated granites, and imply that much larger parental Alice Springs-age granites exist at depth. The mechanism to allow repeated felsic magmatism in an intraplate setting, where all exposed rock types had a previous high-temperature history, is enigmatic. However, we suggest that episodic underthrusting and dehydration of unmetamorphosed Centralian Superbasin sedimentary rocks allowed crustal fertility to maintained over a c. 140 Ma interval during the intra-plate Alice Springs Orogeny. [source] | ||||||||||