Mesozoic alkaline rocks of the Averill plutonic complex

Mesozoic alkaline rocks of the Averill plutonic complex MYRAKEEP^

AND

J. K. RUSSELL

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Department of Geological Sciences, fie University of British Columbia, Vancouver, B. C . , Canada V6T 2B4

Received December 17, 1991 Revision accepted July 8, 1992 The 150 Ma Averill alkaline plutonic complex is situated in southern British Columbia within the Intermontane Belt of the Canadian Cordillera. It comprises concentrically arranged phases of pyroxenite, monzogabbro, monzodiorite, monzonite, and syenite. Gradational changes in the modal composition of the phases contrast with an abrupt change in crystallinity from euhedral mafic minerals in the ultramafic-mafic phases to an anhedral, interstitial habit for mafic minerals in the syenite. Whole-rock compositions have clear alkaline affinities (e.g., feldspathoid normative) and indicate a chemical discontinuity between the ultramafic-mafic phases and the late syenite phases. Melanite garnet is an important accessory mineral of the syenite and is characteristic of silica-undersaturated alkaline intrusions. Clinopyroxene, feldspar, amphibole, biotite, melanite, and titanite compositions compare closely with those of other alkaline complexes in the western Cordillera. Le complexe plutonique alcalin d'Averill, Igt de 150 Ma, est localisk dans le sud de la Colombie-Britannique au sein du Domaine intermontagneux de la Cordillkre au Canada. I1 est form6 d'unites lithologiques dispostes concentriquement, qui sont pyroxtnite, monzogabbro, monzodiorite, monzonite et syknite. Les changements graduels dans la composition modale des unitks lithologiques contrastent avec le changement abrupt de cristallinitk, dans les lithologies ultramafiques-mafiques les minkraux mafiques sont idiomorphes, tandis que dans la syknite les minkraux mafiques sont interstitiels et xtnomorphes. Les compositions chimiques dkterminees sur roche totale prksentent des affinites nettement alcalines (ex., feldspathoide normatif), et elles rkvblent une discontinuitk chimique entre les lithologies ultramafiques-mafiques et les lithologies de syenite tardives. Le grenat mklanite est un mineral accessoire important dans la syknite, il est caractkristique d'intrusions alcalines sous-saturtes en silice. Les compositions de clinopyroxkne, feldspath, amphibole, biotite, mtlanite et titanite se comparent trks bien B celles rencontrkes dans d'autres complexes de la Cordillkre occidentale. [Traduit par la rkdaction] Can. 1. Earth Sci. 29, 2508-2520 (1992)

Introduction In British Columbia, alkaline Mesozoic plutons occur throughout the Intermontane Belt of the Cordillera (e.g., Barr et al. 1976), from the Dease Lake area of northern British Columbia to Princeton in southern British Columbia (Fig. 1). These plutons record a period of magmatic activity in the Cordillera that extended from 200 to 155 Ma (Armstrong 1988). Alkaline plutons deriving from this activity include the Iron Mask, Hogem, Similkameen, Stikine, Guichon, Granite Mountain, and Tulameen batholiths, as well as several other smaller bodies (Armstrong 1988). In addition to their petrologic and tectonic significance, several of these plutons host major porphyry copper deposits, including Copper Mountain Ingerbelle (Fahrni et al. 1976), Cariboo -Bell (Hodgson et al. 1976), Lorraine (Wilkinson 1976), Galore Creek (Allen et al. 1976; Logan and Koyanagi 1989), Mount Milligan, and Afton (Carr and Read 1976). Contemporaneous alkaline magmatism in the United States has been recognized in northern Washington State (Hollister 1976; Fox 1977) and California. The California localities include the White-Inyo Mountains; the Granite Mountains of the southern Mojave Desert and the northern San Bernadino Mountains (Miller 1976, 1977, 1978); the southern Inyo and Panamint Mountains (Dunne 1971; Dunne et al. 1978); the northern White Mountains (Crowder and Ross 1973); the eastern Sierra Nevada (Schweickert 1976); and the Klarnath Mountains (Hotz 1971; Charlton 1977). These Mesozoic alkalic bodies (Fig. 1) define a major Cordilleran magmatic belt in western North America (Armstrong 1988). The Averill alkaline plutonic complex in southern British Columbia is typical of these Mesozoic alkaline plutons. Map'Present address: Department of Geological Sciences, Southern Methodist University, Dallas, Tex. 75275-0395, U.S.A. Printed ~n Canada I lmprlrne au Canada

ping, petrographic, and chemical data for the Averill pluton indicate its alkaline affinity and are similar to data from other well-studied alkaline bodies in British Columbia and the United States. The purpose of this paper is to fully document the mineralogical and chemical characteristics of the Averill pluton, through petrographic observation and whole-rock, mineral, and trace-element chemical analyses.

Regional geology The geology of the Averill plutonic complex is shown in Fig. 2. The oldest rocks, proximal to the Averill complex, are Permo-Carboniferous sandstones, conglomerates, volcanic tuffs, and porphyries of the Franklin Group (Drysdale 1915), which have been recrystallized by regional metamorphism. Mesozoic (Jurassic) calc-alkaline granite intrusions of Nelson and Valhalla affinity intrude the Franklin Group and are themselves intruded by phases of the alkaline Averill plutonic complex (Pinsent and Cannon 1988; Keep and Russell 1989) (Fig. 2). Eocene sedimentary rocks of the Kettle River Formation (Little 1957; Pinsent and Cannon 1988) unconformably overlie the Averill complex rocks. Trachytes of the Marron Volcanics occur at higher elevations in the area and disconformably overlie the Kettle River Formation. North-trending normal faults crosscut the Mesozoic and Eocene rocks (e.g., Parrish et al. 1988). Glacial deposits blanket approximately 85 % of the map area.

The Averill pluton Geology The Averill plutonic complex, first mapped by Drysdale (19151, is a northwest-trending intrusion, approximately 4 km in length, dated at 150 f 5 Ma (K- Ar pyroxene - amphibole age on pyroxenite; R. L. Armstrong, unpublished data). It comprises pyroxenite, monzogabbro, monzodiorite, monzonite, and

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Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by CANADIAN FREE ACCESS 2010 + BACK FILES on 03/27/13 For personal use only.

KEEP AND RUSSELL

A

Towns

8 Belts of alkaline intrusiv

(Copper Mtn. Suite of Woodsworth et al. 1991) Mineral deposits 1- Gnat Lake 2 - Galore Creek-Rugged Mountain 3 - Lorraine 4 - Mt. Milligan 5 - Cariboo-Bell 6 - Afton 7 - Copper Mountain

Individual alkaline intrusions 8 - Averill Complex 9 - Ironside Mountain 10 - E. Sierra Nevada 11- White Mountain 12 - White-Inyo Mountains 13 - Hunter Mountains 14 - Granite Mountains 15 - San Bernadine Mountains 16 - Mt. Lowe

FIG. 1. Map of western North America showing the distribution of Mesozoic belts of alkaline plutonic rocks in British Columbia (shaded) and locations of specific intrusions in British Columbia, Washington, and California (after Barr et al. 1976 and Miller 1978). V, Vancouver; P, Princeton; DL, Dease Lake.

syenite phases and two compositionally distinct sets of late dikes. The intrusion is concentrically zoned, with pyroxenite at the centre, grading outwards through monzogabbro and monzodiorite, to monzonite at the perimeter. Trachytic syenite occurs along the axis of the pluton as a coarse-grained core and a fine-grained marginal phase (Fig. 2). It is mineralogically distinct and is characterized by a prominent alignment of K-feldspar megacrysts. The alignment does not demonstrably relate to the contacts. This may reflect poor sampling due to lack of exposure or a complicated actual pattern. The syenite intrudes the pyroxenite and monzogabbro, and the mafic phases are brecciated along the margin of the syenite. The two sets of dikes which cut the plutonic suite include (i) an altered, buff-coloured, fine-grained trachyte containing alkali feldspar, quartz, and biotite; and (ii) a grey-spotted porphyry comprising remnant plagioclase and chlorite, quartz, calcite, and epidote in a fine-grained groundmass. Both of these dike sets have a north-south to northeast-southwest orientation. The dikes may be coeval, as they are parallel and are not seen to crosscut one another.

Petrography Pyroxenite, monzogabbro, monzodiorite, and monzonite contain the same mineral assemblage and are distinguished solely on the basis of modal proportions (pyroxenite