Showing papers on "Subsolvus published in 1982"

Nature and origin of A-type granites with particular reference to southeastern Australia

Abstract: In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites also occur in Australia and elsewhere. Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates. The melt structure determines the trace element composition of the granite. Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more important than F.

Geochemistry and Rb-sr geochronology of associated proterozoic peralkaline and subalkaline anorogenic granites from Labrador

Abstract: Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common hornblende to actinolite and also coexist with annite-rich biotite.

Petrology and petrogenesis of the Bokan Granite Complex, southeastern Alaska

Abstract: The Bokan Granite Complex is a peralka-line ring-dike complex emplaced into marine shales, volcanic flows and tuffs, and plutonic rocks. Mineralogically and chemically, the Complex is composed of I-type granites. Aegirine- and riebeckite-bearing granite aplites, porphyries, and pegmatites comprise twelve distinct intrusive episodes. The aegirine-bearing rocks occur in an outer annular zone formed during early crystallization. Subsequent rocks are riebeckite-bearing, due to devolatilization of the magma chamber during a collapse-ring-dike emplacement event. Early crystallization of alkali feldspar occurred during magma ascension from a lower crustal-upper mantle source. At shallow depths, subsolvus crystallization allowed microcline and albite to dominate. Local bodies of aegirine syenite were formed during the early collapse-ring-dike emplacement, in response to the magma devolatilization. The riebeckite granites reflect lower Po2 and possibly declining peralkalinity. The granitic rocks at Bokan all exhibit Na2O contents greater than K2O. Lithophile elements are concentrated in all the rocks, especially in zones where hydrothermal albite and chlorite formed. Rb/Sr ratios increase in progressively younger rocks in the Complex. Agpaitic ratios vary from 0.92 to 2.08 for the granitic rocks.