Showing papers on "Hypersolvus published in 2008"

Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications

Abstract: The present study focuses on four A-type fluorite-bearing granitic plutons in the Eastern Desert of Egypt which are classified into post-orogenic subsolvus (Homrit Waggat, 535 Ma; Homer Akarem, 541 Ma and Ineigi, 571 Ma) and anorogenic hypersolvus (Gabal Gharib, 476 Ma) granites. All the granites are Si- and alkali-rich and Mg Ca Ti poor. Whereas both granite types appear relatively homogeneous in terms of most of their major and trace elements, they differ in that the subsolvus granites are depleted in TiO2, FeO*, Ba, Sr and Zr and enriched in Rb and Y with respect to the hypersolvus granites. The two granite types, however, can be distinguished more easily by their rare-earth element (REE) patterns. Chondrite-normalized REE patterns of the hypersolvus granite display a gull-wing shape, characterized by a large negative Eu anomaly and moderate-to-high LREE contents. Relative to the hypersolvus granite, subsolvus granite is depleted in LREE and more enriched in HREE contents. The increase of HREE in the subsolvus granite is presumably caused by F complexing during the late stage of its evolution. This is supported by the abundance of fluorite veins cross-cutting the subsolvus granite. The negative Eu anomalies in the subsolvus granite point to the role of feldspars as residual phase in the source, and as a crystallizing phase during magmatic differentiation. Field relations, textural, mineralogical and geochemical data of the post-orogenic subsolvus granite are consistent with its derivation from a parental basic magma through crystal–liquid fractionation of alkali feldspar, plagioclase, amphibole, Fe Ti oxides, titanite, zircon, monazite and allanite. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of a post-orogenic and an anorogenic setting. On the other hand, the most credible mechanism for the origin of the anorogenic hypersolvus granite is partial melting of I-type granodiorite–monzogranite source rocks in the study area.

Geochemistry and petrogenesis of post-collisional ultrapotassic syenites and granites from southernmost Brazil: the Piquiri Syenite Massif

Abstract: ThePiquiriSyeniteMassif, southernmostBrazil, ispartofthepost-collisionalmagmatismrelatedtotheNeoproterozoic Brasiliano-Pan-African Orogenic Cycle The massif is about 12 km in diameter and is composed of syenites, granites, monzonitic rocks and lamprophyres Diopside-phlogopite, diopside-biotite-augite-calcic-amphibole, are the main ferro-magnesian paragenesis in the syenitic rocks Syenitic and granitic rocks are co-magmatic and related to an ultrapotassic, silica-saturated magmatism Their trace element patterns indicate a probable mantle source modified by previous, subduction-related metasomatism The ultrapotassic granites of this massif were produced by fractional crystallization of syenitic magmas, and may be considered as a particular group of hypersolvus and subsolvus A-type granites Based upon textural, structural and geochemical data most of the syenitic rocks, particularly the fine-grained types, are considered as crystallized liquids, in spite of the abundance of cumulatic layers, schlieren, and compositional banding Most of the studied samples are metaluminous, with K2O/Na2O ratios higher than 2 The ultrapotassic syenitic and lamprophyric rocks in the Piquiri massif are interpreted to have been produced from enriched mantle sources, OIB-type, like most of the post-collisional shoshonitic, sodic alkaline and high-K tholeiitic magmatism in southernmost Brazil The source of the ultrapotassic and lamprophyric magmas is probably the same veined mantle, with abundant phlogopite + apatite + amphibole that reflects a previous subduction-related metasomatism

A-type granites from the nagarparkar complex, pakistan: geochemistry and origin

Abstract: The Neoproterozoic Nagarparkar Complex is characterized by isolated semicircular hillocks of outcrops of "withinplate" A-type peralkaline to peraluminous granites. Geochemically the granites/rhyolites of the Nagarparkar Complex are characterized by high SiO2, Na2O+K2O, K2O, Fe/Mg, Al2O3, Zr, Nb and Y and low CaO and Sr. Majority of samples show moderate iron enrichment. The rocks show the characteristic trend of alkali enrichment and depletion of Ti, Fe, Mg and Ca, which are comparable with the rift-related mildly alkaline suites worldwide. Another characteristic feature of the alkaline/mildly alkaline felsic volcanoplutonics of the Nagarparkar area is their enrichment in incompatible trace elements similar to other peralkaline rocks of the Basin. High values of incompatible trace elements like Zr, Nb, Ce, and Y are characteristic of the granites/rhyolites under study. The multielement primitive mantle normalized spidergrams show that the granites/rhyolite from Nagarparkar Complex are enriched in Rb, Y, U and Zr, a characteristic of A-type granitoids. These granites are also strongly depleted in Ti and Sr. Negative Ti anomaly can be interpreted as reflecting ilmenite fractionation. Nb shows low negative anomaly, which is typical of crustal material. The negative anomaly shown by Ba and Sr can be attributed to low-pressure felspar fractionation and are typical of A-type granitoids. Primitive mantle normalized spider diagrams show that the Nagarparkar granites/rhyolites are enriched in Rb, La, Ce, Zr and Y and depleted in Sr, Ba, Ti, P and show negative Nb anomaly, which is typical of upper crust (hypersolvus). The Chondrite normalized spidergrams show that LREE are enriched as compared with HREE. A wide variation in Eu/Eu*values suggest felspar fractionation. P.I Index of majority of samples varies from .87 to 1. A/CNK versus ANK plots clearly confirm peralkaline to peraluminous nature of the rocks. The presence of Na-amphibole in the granites further confirms the alkaline nature of the granites. Granites/rhyolite plot as "alkali granite" in the QAP diagram. The normative appearance of acmite and nepheline in some samples further supports the alkaline nature of these rocks. The granites/rhyolites of the Nagarparkar Complex exhibit "within plate" setting on plotting in different discrimination diagrams.