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Probable Low-Pressure Intrusion of Gabbro Into Serpentinized Peridotite, Northern California

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Probable Low-Pressure Intrusion of Gabbro Into Serpentinized Peridotite, Northern California Probable low-pressure intrusion of gabbro into serpentinized peridotite, northern California KATHLEEN R. SCHWINDINGER* \ „ , , _ , . _ . TT . „,. ALFRED T ANDERSON Jr J DePartment °fl"e Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 ABSTRACT the correlation between these igneous and metamorphic features, it is likely that the Castle Lake gabbro intruded its own hydrothermal The Castle Lake gabbroic body intrudes the Trinity ultramafic envelope of serpentinized peridotite. sheet in northern California. The extent of concentric reaction zones of talc, tremolite, and chloritic blackwall on included blocks of perido- INTRODUCTION tite correlates with increasing proportions of igneous hornblende in the gabbroic host:; consequently, the reaction zones probably formed Sharply intrusive bodies of massive and layered gabbro characterize during initial cooling of the gabbro. The principal source of water in the Trinity and other Klamath peridotites (Loney and Himmelberg, 1977; the gabbroic magma and in the reaction zones was probably a hy- Goulland, 1977; Throckmorton, 1978; Quick, 1981). In contrast, there is a drothermal system which existed during and after crystallization of gradational transition from tectonized ultramafic rocks, through layered the magma. Blocks of pyroxenite in a succession of intrusive sheets of gabbros, to massive gabbros and diabase dikes in well-preserved ophio- gabbroic rock indicate repeated intrusion of magma into its previously lites, such as Samail (Pallister and Hopson, 1981; Boudier and Coleman, solidified margins. An increasing proportion of hornblende in the 1981), Troodos (Moores and Vine, 1971), and others (Coleman, 1977). younger sheets correlates with a marked increase in the grain size. The layered portions of some of the Klamath gabbroic bodies have con- Variable distribution of dissolved and exsolved water and transitory formable contacts with peridotite (Throckmorton, 1978; Quick, 1981), minor decompression events can explain the grain-size layering in the but the massive gabbros intrude into and include xenoliths of peridotite. younger sheets. Vugs as much as 5 mm in diameter in the pegmatitic Thin lenses and seams of intrusive gabbro are present in many ultramafic hornblende gabbro are interpreted to be igneous in origin and, to- tectonites both in the Klamaths (Dick, 1977; Quick, 1981) and elsewhere gether with the patchiness of the metamorphism, suggest that the (Boudier and Nicolas, 1977; Boudier and Coleman, 1981). It is principally gabbro crystallized at a pressure of ~2 kbar or less. Igneous horn- their thickness, and their intrusive nature, that distinguishes the massive blende (near solidus) and rare garnet are consistent with such a low gabbros of the Klamaths from those in ophiolites such as Samail. We pressure. The lack of sodium-rich plagioclase in the patchily meta- describe and interpret some of the lield relations of the gabbro and perido- morphosed gabbros suggests that the hydrothermal (metamorphic) tite at Castle Lake, in an attempt to establish their conditions of formation. fluids were poor iri sodium (nonmarine or distilled marine). Because of Quick (1981) noted that the Trinity peridotite is more serpentinized near the gabbroic bodies. The spatial association of gabbro and ser- pentinized peridotite might be explained in several ways. (1) Early serpentinization provided a weak zone favorable for intrusion; (2) a hy- drothermal circulation system developed during the emplacement of the pluton; or (3) the pluton, being poor in deformable olivine, fractured under stress and served as permeable conduit for hydrothermal solutions. If the serpentinization was associated with the emplacement and crystallization of the gabbro, then it may be surmised that the gabbro intruded the ULTRAMAFIC fp^l VOLCANIC and BASIC INTRUSIVE ^ SEDIMENTARY GRANITIC INTRUSIVE • UNMAPPED Figure 1. Location map. Castle Lake is 7 mi south-southwest of the city of Mt. Shasta (just beyond the northern edge of the map, along Route 5) and is easily accessible from there. Figure 2 shows the area directly south of Castle Lake (snuill open square). The Figure 1 map is a modification of the Weed Sheet of the Geologic Map of California, including general information from Throckmorton (1978). •Present address: 1935-21G Eastchester Road, Bronx, New York 10461. Geological Society of America Bulletin, v. 98, p. 364-372, 14 figs., 2 tables, March 1987. 364 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/3/364/3445387/i0016-7606-98-3-364.pdf by guest on 29 September 2021 INTRUSION OF GABBRO INTO PERIDOTITE, CALIFORNIA 365 peridotite at a low pressure near a surficial source of water. We think that PETROGRAPHY the detailed field relations at Castle Lake are best explained by such coeval intrusion and serpentinization. Peridotite and Related Rocks LOCATION AND REGIONAL GEOLOGY Serpentinized peridotite and serpentinite occur both as country rocks enclosing, and as inclusions within, the gabbroic rocks. We have studied The Castle Lake gabbroic body is in the eastern part of the Klamath only the inclusions. The serpentine occurs in veins between relic grains of Mountain Province, Siskiyou County, 7 mi south-southwest of Mt. Shasta olivine and has a fibrous mesh texture. There is a concentration of opaques City (Fig. 1). We mapped a gabbroic body, which has excellent exposures, in the center of the veins. Intergrown blades of serpentine, associated with on the north facing, cirque floor and wall, south of Castle Lake between tremolite, are interrupted by and interrupt the veins of mesh serpentine. 5,600 and 6,200 ft elevation. The gabbroic body is in the Trinity ultra- Primary pyroxene is absent, but millimetre-sized, twinned crystals of ac- mafic complex and is bordered on the northwest by the Castle Crag quartz tinolite, with included magnetite, compose 5% to 30% of most inclusions. diorite stock and on the south by glacial deposits. Talc and microscopic prisms of olivine occur in some of the serpentinite The Klamath Mountains are interpreted to consist of remnants of veins (Fig. 4). Some of the olivines in the inclusions have kink bands; in oceanic crust and island arcs, accreted sequentially from east to west some cases, arrays of 10- to 100-/L¿ diameter olivine grains are aligned during the Paleozoic and Mesozoic Eras (Irwin, 1981). The Trinity ultra- along former kink-band boundaries. Many of the inclusions have concen- mafic complex, like many of the other ultramafic bodies in the Klamath tric, monomineralic zones of talc and tremolite (Figs. 5 to 8). Mountains, is arcuate (concave to the east), elongate, and concordant with the surrounding rocks (Irwin, 1960; Irwin and Lipman, 1962). The gab- Gabbroic Rocks broic plutons in the Trinity peridotite are confined to the ultramafic bodies (Lipman, 1964; Hotz, 1971), and the intrusion of the gabbroic All of the gabbroic rocks are partially metamorphosed. Plagioclase is rocks predates the thrusting of the peridotite against the Central Metamor- partly to wholly replaced by chlorite and zoisite, clinozoisite, or epidote, phic belt (Goulland, 1977). U-Pb dates of layered gabbros range from and pyroxene is partly to wholly replaced by actinolite, chlorite, and/or 455-480 m.y.; a U-Pb date of a massive, pegmatitic gabbro is 430 m.y. hornblende (Fig. 9). Original textures are preserved. We use rock names (Lanphere and others, 1968; Mattison and Hopson, 1972). on the basis of the inferred, initial (premetamorphism) mineralogy because the identities and textures of the original minerals are apparent in the field. FIELD RELATIONS Pyroxenite/Gabbro The field relations of mafic and ultramafic rocks at Castle Lake have been mapped by ourselves and by Throckmorton (1978). Gabbroic rock The average grain size of pyroxenite and gabbro is < 1 cm. Pyroxenite crops out in ~6 mi2 of a 14-mi2 area (Fig. 1) which is mostly west of (Fig. 10) is more abundant than is gabbro. It is characterized by having Castle Lake (Throckmorton, 1978). We have mapped, in detail, -2% of <20% interstitial, anhedral patches of plagioclase (or its metamorphosed the gabbroic body (Fig. 2). There is a rough correspondence, with great equivalent), whereas gabbro (Fig. 7) has 20% ophitic to 50% subhedral variability, between our lithologic units and those of Throckmorton. Our plagioclase (or its metamorphosed equivalent). Most of the relict pyroxene lithologic units are peridotite (serpentinized peridotite of Throckmorton), is clinopyroxene. Some of the gabbros have deep, red-weathering pits, pyroxenite/gabbro (cumulus ultramafic rock and layered gabbro), pyrox- suggestive of former olivine. Some of the gabbros and pyroxenites have as enite with peridotite blocks (cumulus ultramafic rock and serpentinized much as 5% quartz. Throckmorton (1978) reported microprobe analyses peridotite), pegmatitic gabbro and pegmatitic hornblende gabbro (massive of plagioclase (An 90-96), pyroxenes, and olivines (Fo 84-92). and intrusive gabbros), hornblende diorite (not recognized), and quartz diorite (Castle Crags diorite pluton and associated dikes and plugs). Pegmatitic Gabbro and Pegmatitic Hornblende Gabbro The gabbroic rocks are intrusive into the peridotite and decrease in areal extent with increasing proportion of hornblende. Roughly visualized, Although the grain size of the pegmatitic rocks ranges from 1 to Throckmorton's map (1978) shows cumulus ultramafic rocks on the east 30 cm, most grains are < 10 cm. The petrography of the pegmatitic rocks is and serpentinized
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