Two-mica granites; Part A, Their occurrence and petrography

Abstract

Classical European Hercynian two-mica granites host or are genetically related to tin, tungsten and uranium deposits. Similar rocks elsewhere in the world are associated with similar ore deposits, and a knowledge of two-mica granites can be a useful exploration tool.Two-mica granites occur usually as small plutons late in a plutonic sequence that intrudes high-grade metamorphic rocks of an orogen. In plan they are elongate or eliptical and in three dimensions they are tabular or sheet-like. They are usually leucocratic, massive, hypidiomorphic granular granitoids that consist of about 31 volume percent quartz, 26 percent K-spar, 31 percent plagioclase and about 10 percent mica in which muscovite may exceed biotite. In some, muscovite is absent in which event total mica is about 5 percent, except for a few types that contain abundant biotite. In two-mica granites hornblende is absent, magnetite is sparse, and sphene is rare. Common accessory minerals are apatite, zircon, monazite, garnet, and tourmaline.Two-mica granitoids are more similar to S-type and ilmenite-series granitoids of Chappell , and White (1974) and Ishihara (1977), respectively, than to their I-type and magnetite-series granitoids. Modal plots of two-mica granitoids fall predominantly in the monzogranite subfield of Streckeisen's (1976) modal classification of granitoids. Modal plots, however, do not uniquely define two-mica granites.

Lithology and fracture characterization from drilling investigations in the Mirror Lake area, Grafton County, New Hampshire

“…Geologic names were not assigned to the rocks in the boreholes but the schists probably correspond to the metamorphic rocks of the Silurian-age Rangeley Formation (Lyons and others, 1997). The granite is most likely anatectic, two-mica granite of the Devonian Concord Granite (Armstrong and Boudette, 1984;Lyons and others, 1982;J.B. Lyons, Dartmouth College, written commun., 1994).…”

“…The host rock was intruded by granitoids (felsic igneous intrusions), pegmatites, and diabase. The granitoids in the eastern part of the study area are predominantly anatectic, two-mica granite of the Devonian Concord Granite (370-365 Ma) (Armstrong and Boudette, 1984;Lyons and others, 1997). The granitoids also include gneissic textured Bethlehem Granodiorite (Lyons and others, 1997), which was previously referred to as Bethlehem Gneiss (Billings, 1956).…”

“…Commonly two-mica granites are found in sheet-like, tabular bodies that have been intruded along preferred zones of weakness that conform to the fabric of the host rock. These tabular bodies occasionally exhibit weak foliation comprised of biotite and muscovite, and sometimes contain strongly foliated biotite schlieren, which are oriented parallel to major structural features in the host rock (Armstrong and Boudette, 1984).…”

U-Pb Detrital Zircon Geochronology within the Cape Fold BeltKaroo Basin System

“…55). Topaz is a relatively common mineral found in granites which may two-mica grainites" which contain both muscovite and biotite with low amounts of hornblende [Armstrong and Boudette, 1984]. There is little evidence suggesting the weathering of other sedimentary rocks or metamorphic rocks due to the low amounts of metamorphic mineral phases.…”

Application of Seismic Radial Anisotropy for Near-Surface Fractures Identification

“…Granitoids, pegmatites, and diabase later intruded it. The Concord Granite was intruded in the Late Devonian (370-365 Ma) (Armstrong and Boudette 1984) and included fingers and dikes, which are both parallel and crosscut the foliation of the schist. The bedrock and these granite intrusions were subsequently intruded by pegmatite in the Late Devonian (Ellefsen et al 2002).…”