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Geologic Map of the Humboldt Lopolith Surrounding Terrane

Geologic Map of the Humboldt Lopolith Surrounding Terrane

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Full Title: Geologic Map of the Humboldt Lopolith and Surrounding Terrane, Nevada

Compiler: R.C. Speed


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The Humboldt lopolith, an igneous complex of Middle Jurassic age, underlies an area of about 1,000 km2 and occupies a volume of between 1,700 and 2,500 km3 in the western Basin and Range province. Intrusive rocks constitute the lopolith, except for a cap of volcanic rocks in the central part of the complex. The lopolith almost exclusively intrudes or is in depositional contact with a suite of Jurassic sedimentary rocks that are the youngest marine deposits of the region. The lopolith and its Jurassic wall rocks lie, with thrust contact, above thick basinal pelitic rocks of early Mesozoic age. Pelitic rocks are intruded by the lopolith only at its western margin, where a nappe of pelite is included within the terrane of allochthonous wall rocks. Regionally, the lopolith, its wall rocks, and the subjacent pelitic rocks constitute the Fencernaker allochthon, which was thrust over early Mesozoic shelf deposits during or after emplacement of the lopolith. Volcanic rocks of the lopolith consist of basaltic lava, breccia, and dikes, together with basalt-derived sedimentary rocks. The upper part of the volcanic cap consists of chiefly stratified rocks and the lower part of chiefly massive rocks. Intrusive rocks are of two major compositional groups: gabbroic (gabbro, picrite, and anorthosite) and sodic (keratophyre, albitite, scapolitite, and diorite). Layering and igneous foliation are well developed in intrusive rocks of the peripheral zone. There, layered rocks occur either as thick bistratal sheets of picrite and anorthosite or as finely rhythmic alternations of mafic and felsic gabbro. In the interior, gabbroic rocks are generally unlayered and grade up to sodic rocks within 100 to 300 m of the intrusive roof.

Published: 1/03/1977

Pages: 4, plus 2 sheets

Product Category: Maps and Charts