Abstract

The Mid-Triassic Red Hill syenitic complex, a member of the White Mountain magma series, s i an oval-shaped multiple intrusion. The country rock has been brecciated and contact-metamorphosed near the margin. Six major rock units—five of syenitic composition and one of granite—compose the complex. The oldest unit, the outer coarse syenite, consists mainly of perthite and ferrohastingsite, and is one of three concentrically arranged units. This rock type changes abruptly inward to a zone of nepheline-sodalite syenite. These two units possess a cumulate feldspar lamination that occurs in zones and forms an inward-dipping, centripetal dip pattern of 60° to 85° around the complex. The original cumulate texture in these rocks has been partially erased by auto-metamorphism. The central portion of the complex is comprised of the younger, non-foliated fire tower syenite, which has been intruded by two small, plug-like bodies of quartz syenite, and one small intrusion of granite. The five syenitic rock units show clustered properties of being slightly silica-deficient and of high alkali and iron content. Their compositions are representative of a low-temperature residual magma which could yield both feldspathoidal- and quartz-bearing rocks. The rocks of the complex are estimated to have been emplaced by ring-fracturing and block subsidence under a lithostatic pressure of 1.5 kb and a temperature range of 680° to 750° C.

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