UBC Theses and Dissertations

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UBC Theses and Dissertations

The spuzzum pluton Northwest of Hope, B.C. Vining, Mark Richard


The Spuzzum Batholith underlies an area northwest of Hope, B.C. It is nearly 60 km. long in a northerly direction and 10 to 20 km. across. The southern part of the body is zoned from pyroxene diorite in its core, through hornblende diorite, to biotite-hornblende tonalite. Tonalite forms about two thirds of the pluton's area, forming a nearly continuous rim. The pluton intrudes upper Paleozic Chilliwack Group, the Cretaceous (?) Giant Mascot Ultramafic Body, and Settler Schist of unknown age. K-Ar ages for tonalite and diorite range from 76 to 103 m.y. Diorite consists of subhedral orthopyroxene and plagioclase (An₆₂ to An⁴¹), with variable amounts of hornblende and clinopyroxene. Tonalite is largely composed of anhedral quartz and biotite, and subhedral hornblende and plagioclase (An⁵⁰ to An₃₂). Tonalite and some diorites are foliated. These rocks are locally hornblendized, resembling hornblende gabbro. Pods of directionless hornblendite are common in hornblendized rocks. Foliations and mineralogical zonation outline a crude tongue-like structure, modified by later deformation. Spuzzum diorite appears to have intruded the main part of the Giant Mascot Ultramafic Body but some hornblendites are younger than diorite. The Giant Mascot Ultramafic Body, 2 by 3 km., is zoned from dunite or peridotite, through pyroxenite, to a rim of hornblendite up to 100 m. across. Hornblendite occurs also as dykes. Orthopyroxenes of Spuzzum diorite are weakly aluminous hypersthene; those of the contact with Giant Mascot pyroxenite are bronzite. Clinopyroxenes of the same rocks are somewhat more aluminous salites and diopsides. Hornblendites from Spuzzum diorite and the Giant Mascot Ultramafic Body resemble alkali basalt in composition. Hornblende analyses fall into three categories: edenite, pargasite-common hornblende, and thirdly, more iron-rich common hornblende. It is concluded that Spuzzum diorite and tonalite originated by crystal settling from quartz diorite magma at depth, followed by diapiric rise of a zoned pluton composed of residual tonalite liquid cored by drawn-up dioritic cumulate. A mathematical test shows the compositions of diorite and tonalite to be consistent with this hypothesis. The rising pluton subsequently engulfed the Giant Mascot Ultramafic Body. Hornblendites may have formed by metasomatism of these rocks and adjacent diorite or tonalite as a consequence of the second boiling of tonalite and the coursing of resultant hydrothermal fluid through the nearly solid pluton.

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