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Retrograde P (retrograde + p)
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] Phase equilibria modelling of kyanite-bearing anatectic paragneisses from the central Grenville ProvinceJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2008A. INDARES Abstract Kyanite-bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high- P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite,garnet ± kyanite ± plagioclase ± K-feldspar,quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na2O,CaO,K2O,FeO,MgO,Al2O3,SiO2,H2O,TiO2,O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub-aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P,T conditions of the metamorphic peak. These P,T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high- T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P,T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P,T conditions (14.5,15.5 kbar and 840,890 °C) and steep retrograde P,T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P,T conditions of 12.5,13 kbar and 815,830 °C and flatter P,T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P,T evolution of high- P anatectic quartzo-feldspathic rocks. [source] P,T,fluid evolution in the Mahalapye Complex, Limpopo high-grade terrane, eastern BotswanaJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2005K. HISADA Abstract Metapelites, migmatites and granites from the c. 2 Ga Mahalapye Complex have been studied for determining the P,T,fluid influence on mineral assemblages and local equilibrium compositions in the rocks from the extreme southwestern part of the Central Zone of the Limpopo high-grade terrane in Botswana. It was found that fluid infiltration played a leading role in the formation of the rocks. This conclusion is based on both well-developed textures inferred to record metasomatic reactions, such as Bt , And + Qtz + (K2O) and Bt ± Qtz , Sil + Kfs + Ms ± Pl, and zonation of Ms | Bt + Qtz | And + Qtz and Grt | Crd | Pl | Kfs + Qtz reflecting a perfect mobility (Korzhinskii terminology) of some chemical components. The conclusion is also supported by the results of a fluid inclusion study. CO2 and H2O ( = 0.6) are the major components of the fluid. The fluid has been trapped synchronously along the retrograde P,T path. The P,T path was derived using mineral thermobarometry and a combination of mineral thermometry and fluid inclusion density data. The Mahalapye Complex experienced low-pressure granulite facies metamorphism with a retrograde evolution from 770 °C and 5.5 kbar to 560 °C and 2 kbar, presumably at c. 2 Ga. [source] Pro- and retrograde P,T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implicationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2004A. Zeh Abstract Interpretations based on quantitative phase diagrams in the system CaO,Na2O,K2O,TiO2,MnO,FeO,MgO,Al2O3,SiO2,H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P,T path. That path displays a prograde P,T increase from 600 °C/7.0 kbar to 780 °C/9,10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P,T decrease to 600 °C/4 kbar. The data used to construct the P,T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al-rich garnet,biotite,cordierite,sillimanite,K-feldspar,plagioclase,quartz,graphite gneiss (2) a K-poor, Al-rich garnet,biotite,staurolite,cordierite,kyanite,sillimanite,plagioclase,quartz,rutile gneiss, and (3) a K, Al-poor, Fe-rich garnet,orthopyroxene,biotite,chlorite,plagioclase,quartz,rutile,ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro- and retrograde P,T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983). [source] | ||||||||||