Highwood Mountains

The igneous rocks of the Highwood Mountains occupy an area of about 50x50 km and represent the deeply eroded remnants of a volcanic centre of flows, a dyke swarm, stocks and laccoliths, one of the last being Shonkin Sag which is described independently (No. 30). The igneous rocks lie upon, or are intruded into, Cretaceous sediments. Local uplift and erosion of the sediments were followed by initial extrusion of tuffs, breccias and flows of rather uniform quartz latites characterized by phenocrysts of plagioclase, hornblende and augite. The latites were then considerably reduced by a long period of erosion before volcanism was renewed, but this time with a strongly alkaline character. Flows and pyroclastics in about equal volumes were extruded, the maximum remaining thickness being about 650 m. The principal rock type was given the portmanteau term 'mafic phonolite' by Larsen (1941) and apart from in situ differentiation much the same magma seems to have produced the intrusions and the extrusive rocks. The extrusive rocks carry as their main minerals orthoclase, nepheline, leucite, pseudoleucite, analcime, olivine, augite and biotite, and they are commonly highly vesiculated rocks, the vesicles being more or less filled with zeolites. Nearly all of them are porphyritic, the phenocrysts normally forming about half the rock. Augite phenocrysts, sometimes with a green rim, make up about 20% of most rocks, olivine phenocrysts are normally fresh, analcime may be clear or cloudy and leucite phenocrysts may be fresh or altered to pseudoleucite; biotite and sanidine phenocrysts are sparse. The groundmass is mostly analcime, with small rods of augite, a little olivine, mostly altered, magnetite, apatite and a variable amount of sanidine. The rocks were subdivided by Larsen (1941, p. 1743) as follows: (a) mafic leucite phonolite, with fresh leucite; (b) mafic analcime phonolite, in which the analcime may be clear, cloudy, or altered to natrolite and other zeolites; (c) mafic pseudoleucite phonolite with either pseudoanalcime or leucite, now sanidine and analcime; (d) mafic phonolites, without phenocrysts of analcime or leucite and (e) miscellaneous basaltic rocks, without olivine, leucite or analcime; these are scarce, constituting less than 4% of the rocks examined. Detailed petrographic descriptions and analyses of these varieties willbe found in Larsen (1941, pp. 1743 and 1749). The dyke swarm is essentially radial, this being particularly noticeable on the northern and eastern sides of the mountains where they are more abundant. The dykes vary from about 0. 6 m to 6 m in thickness but can rarely be followed for more than about 1. 5 km. They are very varied in form and composite and multiple types are found. Some irregular bodies were called 'chonoliths' by Buie (1941, p. 1778), and a differentiated dyke with a rounded, bulbous head, called the 'Headed Dike', has been described in particular detail by Buie (1941, p. 1780). Four basic types can be distinguished. (1 )Amphibole phonolites are rare, the amphibole being hornblende and they are probably related to the quartz latites. (2) Pyroxene and pseudoleucite mafic phonolites grade into each other and constitute about half of all the dykes. They contain up to 27% diopside, some olivine, sanidine, interstitial nepheline and a little biotite; in about half the rocks pseudoleucite is found and, in a few, fresh leucite. (3) Biotite phonolites constitute about 25% of the dykes. They contain phenocrysts of biotite, augite and olivine in a groundmass of alkali feldspar, altered nepheline and possible pseudoleucite. (4) Syenite porphyries only form a small proportion of the dykes and were the last to be emplaced. They contain abundant phenocrysts of sanidine and a few of biotite and augite, some with green rims, in a groundmass of alkali feldspar, aegirine and accessories. The Headed Dike is nearly 5 km in length and up to 45 m wide. The dyke has differentiated in situ, the greater part being of shonkinite with about 40% of pyroxene, 34% alkali feldspar and 10% zeolite after nepheline, but the uppermost few m are of fergusite consisting of 55% pseudoleucite, 25% sanidine and only 10% pyroxene. The irregular intrusive bodies described as chonoliths are principally of biotite phonolite, but the Haystack Butte in the northeast of the area consists of monticellite peridotite, with about 20% olivine, 32% monticellite and 22% biotite, and a melilite-nepheline-biotite alnoite or turjaite. Ba-rich phlogopite from Haystack Butte has been described by Wendlandt (1977). Six stocks occur in the Highwood Mountains composed mainly of shonkinite and mafic syenite, with one of monzonite and later felsic syenite; one stock is the type locality for fergusite. The mineralogy is variable but predominantly of sanidine, augite, pseudoleucite and rare leucite, with some biotite and olivine. The stocks are described in detail by Burgess (1941). The laccoliths lie north and east of the volcanic mountains and all but one have been intruded into the Upper Cretaceous Eagle sandstone. There are nine in all, one of which, Square Butte, rises over 600 m above the surrounding plains. All are petrographically similar having shonkinite as their principal rock type, a rock first described from the Square Butte laccolith, and they also usually contain a syenite phase at a higher level in the intrusion. The Shonkin Sag laccolith displays more complex relationships and is described in the next entry. The petrography of the Square Butte laccolith is typical of the others. The lower half is a shonkinite of 50% augite, 13% biotite, 5% olivine, 30% sanidine, nepheline and secondary zeolites and 2% apatite, magnetite and sphene; masses of pseudoleucite are occasionally to be found. The overlying syenite is qualitatively similar but with a higher proportion of felsic minerals. About 80 m below the upper contact olivine becomes uncommon and amphibole replaces pyroxene, and in the uppermost 30 m the rock is a sodalite-hornblende syenite with a little nepheline, analcime and albite. Full descriptions, including maps of the laccoliths, will be found in Hurlbut and Griggs (1939). An interpretation of differentiation in Square Butte in terms of liquid immiscibility is given by Kendrick and Edmond (1981). Metamorphism associated with the intrusive rocks of the Highwood Mountains is extensive and involves hornfelsing and metasomatism, some akin to fenitization. It is described by Larsen and Buie (1941). Accounts of the mineralogy and petrology of the igneous rocks are to be found in Larsen et al. (1941a and b), and data on Sr isotopes in Powell and Bell (1970). Palaeomagnetic data are available in Diehl et al. (1983).