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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

Probable low-pressure intrusion of into serpentinized , 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 , tremolite, and chloritic blackwall on included blocks of perido- INTRODUCTION tite correlates with increasing proportions of igneous 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 (Loney and Himmelberg, 1977; the gabbroic 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 in a succession of intrusive sheets of , to massive gabbros and dikes in well-preserved ophio- gabbroic 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 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 , 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 are consistent with such a low gabbros of the Klamaths from those in such as Samail. We pressure. The lack of sodium-rich 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 , 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

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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 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 is absent, but millimetre-sized, twinned crystals of ac- complex and is bordered on the northwest by the Castle Crag tinolite, with included , compose 5% to 30% of most inclusions. stock and on the south by glacial deposits. Talc and microscopic prisms of olivine occur in some of the serpentinite The are interpreted to consist of remnants of veins (Fig. 4). Some of the in the inclusions have kink bands; in 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 , 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- pits, pyroxenite/gabbro (cumulus and layered gabbro), pyrox- suggestive of former olivine. Some of the gabbros and 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), , 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 peridotite (tectonite) on the west. The relationship be- summarized in Table 1. Hornblende occurs interstitially or as subhedral tween the two ultramafic units was not established by Throckmorton. rims of variable thickness on cores of pyroxene (or its metamorphosed Modal layering in the gabbroic rocks dips moderately in a northerly direc- equivalent) (Fig. 11). Pegmatitic hornblende gabbro has more hornblende tion and is not, in general, parallel to the contact between the gabbro and than pyroxene. With an increase in proportion of hornblende to pyroxene, the ultramafic units (Throckmorton, 1978). Pyroxenite/gabbro and peg- there is a rough increase in the concentration of opaque oxides (almost matitic gabbros occur between the two ultramafic units. Pegmatitic gabbro exclusively , but with minor intergrown rutile and probable pseu- occurs to the west of the pyroxenite/gabbro, but with an inferred contact. dobrookite, and some sulfides), quartz, and rare apatite. Rare, anhedral The intrusive pegmatitic gabbros are reportedly widespread, but they are olivine grains are present. There is 50% to 90% ophitic to euhedral plagio- mappable only in a well-exposed area (Fig. 2) at the eastern contact of clase. The anorthitic plagioclase reported by Throckmorton (1978) is con- layered gabbro, adjacent to cumulus ultramafic rocks (Throckmorton, sistent with our universal stage measurements, except for some zoned 1978). The intrusive gabbros occur as discordant blobs, lenses, and sheets crystals which have irregular, clear rims of approximately An 60. Some of of pegmatitic gabbro in a matrix of pyroxenite/gabbro, with inclusions of the pegmatitic hornblende gabbro has a graphic texture (Fig. 12). peridotite. The younger intrusive pegmatitic gabbros become thinner, richer in hornblende relative to pyroxene, and generally coarser in grain Hornblende Diorite size. The interrelationships of the rock units are schematically summarized in Figure 3. Some of the complexity and irregular contacts in Figure 2 Hornblende diorite (Fig. 13) has skeletal crystals of hornblende as reflect a topographic effect. much as 10 cm long, set in a matrix of 20% to 80% oscillatory zoned

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 2 Figure 2. The geologic map covers 0.3 km of the gabbroic body at Castle Lake. The photographed outcrops can be found by reference to the outlet of Heart Lake, which forms mapped area is in the SE% of sec. 24, T. 39 N., R. 5 W., Dunsmuir (1:62,500) quadrangle, the northeast-trending valley in grid G-9. The northernmost extremity of Heart Lake is in California. The blank areas are Quaternary deposits, mostly glacial till. Locations of the grid G-10.

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Figure 3. Schematic diagram of the age and contact relationships t,sp at,sp J \ ¿? within the Castle Lake gabbro. Serpentinite and serpentinized perido- tite (pd) blocks are included in all of the gabbroic rocks. Although the Çsp ) pyroxenite/gabbro (pxt/gb) is conformable with the pegmatitic gab- bro (pg) in some outcrops (lack of a solid line between the rock units), c most of the pegmatitic gabbro is intrusive (solid lines between map CT>)sP n r r^ v units). Pegmatitic hornblende gabbro (pa), which grades conformably /at.tc J> C^i ¿7 from pegmatitic gabbro, in most cases occurs as intrusive sheets. Pegmatitic hornblende gabbro grades conformably into hornblende ol.sp,tc diorite (ap), which intrudes all other rock types.

£ol,spj J f ol ,tcjsp plagioclase (An 68 to An 20) (or its metamorphic equivalent) plus blue- an J green hornblende, magnetite, iimenite, varying amounts of quartz, apatite, A-v ; sp and small amounts of garnet, epidote, and pyrite. Some of the altered plagioclase grains have rims of clear, albitic plagioclase (An 20). The quartz has vapor-poor fluid inclusions. The plagioclase in the hornblende Figure 4. Photomicrograph (plane light) of serpentinite block diorite that is gradational with the pegmatitic hornblende gabbro is more containing small tabular olivine crystals (ol), talc (tc), tremolite (at), calcic than it is in the sharply discordant hornblende diorite. and serpentine (sp) (grid C-9). (The major constituent minerals of each Some rocks transitional between pegmatitic hornblende gabbro and area are shown in the drawing.) Similar textures also occur in some hornblende diorite have 1% angular, empty to filled, millimetre-sized vugs. serpentinized peridotite blocks. Bar scale is 100 y.. Typical vugs include an 8-mm by 4-mm by 3-mm cavity in a slightly weathered joint surface and a 5-mm by 2-mm by 1-mm cavity in a freshly sawn surface. The cavities are lined with multiple crystals, including plagi- oclase with tiny grains (probably clinozoisite) attached to its facets (Fig. 14). Some of the hornblende diorite rocks have interstitial, empty or in the enclosing gabbroic rock. Inclusions of partially serpentinized perido- filled vugs. tite may be mantled by a zone of tremolite. Most of the tremolite zones are associated with the presence of interstitial and pyroxene-rimming horn- Plagioclase Porphyry blende in the pegmatitic rocks. If talc is present, it usually occurs as a zone between a tremolite zone and a core of serpentinite (Fig. 7). The thickness Although it occurs only as sheets thinner than ~30 cm and is not of both the talc and the tremolite zones increases as the proportion of mapped separately, plagioclase porphyry comprises marginal parts of hornblende increases in the enclosing gabbroic rock (Fig. 7 versus Fig. 8). bodies of gabbro, pegmatitic gabbro, and pegmatitic hornblende gabbro. It The correlation of minéralogie zones and the enclosing gabbroic rock is has 1- to 8-mm-long, euhedral to subhedral plagioclase crystals in a matrix summarized in Table 2. A zone of clinopyroxenite may be present in the of 0.5- to 3-mm plagioclase, pyroxene (or its metamorphic gabbro around some of the peridotite inclusions, and chloritic black wall is equivalent), and hornblende (with actinolite cores). The plagioclase is present adjacent to zoned inclusions in some of the gabbroic rocks (Figs. 6 anorthitic. Approximately 5% iimenite and 0.1% sulfides (pyrrhotite, chal- and 8). The proportion of peridotite inclusions decreases from 90% in the copyrite, pyrite) are present in the fine-grained matrix. pyroxenite/gabbro to < 10% in the pegmatitic hornblende gabbro. The serpentinized peridotite inclusions have various orientations and PERIDOTITE INCLUSIONS AND REACTION ZONES structures. Many of the inclusions are elongate parallel to a fabric of knobby streaks of pyroxene and bluish-weathering seams of serpentine. The minéralogie zonation within the peridotite inclusions correlates The orientation of the seams varies from block to block. There is little or with the degree of serpentinization and with the proportion of hornblende no foliation or lineation of minerals in the enclosing gabbroic rock.

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Figure 5. Adjacent inclusions of ser- pentinite (sp) and serpentinized peridotite (pd) in chloritized pyroxenite (pxt). One block consists of both serpentinized peri- dotite and serpentinite (lower right corner). The pyroxenite is cut by pegma- titiic gabbro (pg), which has thin horn- blende rims on the pyroxenes. A finer grained sheet, richer in , occurs in the pegmatitic gabbro. The lens cap is 5.5 cm in diameter.

Figure 6. Subangular blocks of knobby, serpentinized peridotite (pd) with a rnonomineralic rim of talc (tc), in pegma- titic gabbro (pg) (probably grid G-7). Chloritic blackwall (cb) occurs in the pegmatitic gabbro adjacent to the ultra- mafic block. The compass case on the right is 10 by 6.5 cm.

Figure 7. Serpentinized peridotite (pd) inclusion with rnonomineralic zones of talc (tc) (the large expanse of talc is a geometric artifact of exposure) and tremo- lite (at) in chloritized pyroxenite/gabbro (pxt). This outcrop is south of the mapped area. Scale is indicated by the dime on the left.

Figure 8. Ultramafic inclusion with monomineral zones of talc (tc) and tremolite (at) in hornblende-bearing peg- matitic gabbro (pg) (grid P-4). Chloritic blackwall (cb) extends 15 cm into the pegmatitic gabbro. The scale on the book is 15 cm long.

METAMORPHISM OF THE GAUBROIC ROCKS

The gabbrc ic rocks are patchily metamorphosed to - and TABLE 1. PETROGRAPHY OF GABBROIC ROCKS perhaps, locally, to -grade assemblages. Locally, some out- crops have much relict calcic plagiocliise and pyroxene, whereas other Grain Percentage Hornblende size hornblende* texture outcrops lack either relict mineral or have relics of only one of the miner- als. The size of the metamorphic patches ranges from millimetres to Pyroxenite/gabbro <1 cm None (Fig. 10)

metres. Plagiocliise grains within skeletal hornblende are less altered than Pegmatitic gabbro 1-30 cm <50% Interstitial and (Fig. 1!) those outside of Ihe hornblende. On a larger scale, slightly metamorphosed rims on pyroxene pyroxenite, gabbro, and pegmatitic gabbro occur together, and extensively Pegmatitic hornblende 1-30 cm >50% Rims on pyroxene (Fig. 12) gabbro and subhedral metamorphosed gabbro and pegmatitic gabbro occur together. The meta- morphism cuts across the boundaries between different lithologic units. Hornblende diorite 1-10 cm 100% Skeletal (Fig. 13) The amount of metamorphism of pyroxene and plagioclase increases with Plagioclase porphyry 0.1-0.8 cm Variable Rims on pyroxene proximity to fractures (Fig. 9) but not with proximity to the quartz diorite •Percentage of hornblende to pyroxene. pluton.

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 Figure 9. Sketch of partially metamorphosed pegmatitic gabbro Figure 10. Gradational contacts of py- (grid L-3) with hornblende (hb) rim about a core of pyroxene (py) that roxenite to gabbro to pegmatitic gabbro (grid is partially metamorphosed to actinolite (at). Large subhedral laths of C-4). The pyroxenite, containing 90% sub- plagioclase (pi) are incipiently to extensively metamorphosed to epi- hedral pyroxene, grades to gabbro by an in- dote (ep) and chlorite (cl), especially near quartz-filled (qz) fractures. crease in the proportion of interstitial Anhedral interstitial plagioclase is relatively unaltered. Bar scale is plagioclase (lower right). The segregation- 100 ix. like veins of pegmatitic gabbro have 1-2 cm subhedral pyroxene and plagioclase crystals. A 5-cm rectangular block of talc occurs in the pyroxenite. The scale on the book is 15 cm long.

Figure 12. Massive pegmatitic horn- Figure 13. Hornblende diorite contain- Figure 11. Pegmatitic gabbro containing blende gabbro, having graphic texture and ing 1- to 10-cm skeletal, euhedral hornblende 1- to 4-cm pyroxene which has both thin rims grain-sized layering (grid J-9). The 1- to 4-cm crystals (grid 1-7). The hornblende crystals of black hornblende and interstitial horn- mafic silicates are >80% amphibole. The pho- are randomly oriented in this vein. The com- blende (grid 1-6). Scale indicated by dime. tograph is 25 cm long. pass case is 6.5 cm across.

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Figure 14. Stereo pair photographs of a vug in hornblende gabbro. The vug is in the lower right quarter of the field, which is ~3 cm on a side. The vug is ~8 mm across and contains a millimetre-sized faceted crystal of plagioclase evident in the stereo view.

STRUCTURAL RELATIONS OF THE GABBROIC ROCKS TABLE 2. RELATIONS BETWEEN HOST ROCKS AND ZONES ON ULTRAMAFIC BLOCKS

Serpentinizeti Talc Tremolite The various gabbroic rocks have bob sharp and commonly discor- peridotite dant contacts as well as gradational, conformable contacts. Sharp intrusive contacts reveal a correlation between the age and petrography of the Pyroxenite/gabbro Common Occasionally Rare (Fig. 5) (Fig. 10) (Fig. 7) gabbroic rocks (Fig. 3). The oldest gabbroic rocks, pyroxenite and gabbro, Pegmatitic gabbro* Common Common Occasionally are intruded by the pegmatitic gabbro. The pegmatitic hornblende gabbro (Fig. 6)

intrudes both of the older gabbroic rock units. Hornblende diorite veins Pegmatitic gabbro and Occasionally Common Common intrude all other gabbroic and ultramafic rock units. Successively younger pegmatitic hornblende in large (Fig. 8) gabbro^ blocks gabbroic rocks tend to have sharper, smoother contacts and to occur in

thinner, sheet-like bodies. The same age sequence is evident in the con- •Pegmatitic gabbro with interstitial black hornblende, formable contacts. Pyroxenite grades to gabbro by an increase in the t Pegmatitic gabbros with well-developed rims of horr blende on pyroxene. proportion of plagioclase, then to pegmatitic gabbro by an increase in grain size (Table 1 and Fig. 10). Pegmatitic gabbro grades to pegmatitic hornblende gabbro by an increase in the thickness of the hornblende rims broic rocks. The peridotite was repeatedly invaded by thin seams of gab- on the pyroxene grains. Pegmatitic hornblende gabbro grades to horn- broic magma; the blocks were gradually displaced, but not suspended, by blende diorite by change from anhedrs.l to euhedral hornblende. The the magma through which they would have fallen. gradations occur both on a scale of thin segregation-like veins and over a Reaction of the gabbroic liquid with serpentinized peridotite can distance of metres. The quartz diorite is distinct in having only sharp, explain the localization of the pegmatitic gabbros and the hornblende relatively smooth, discordant contacts and in having a fine-grained matrix diorite adjacent to the peridotite. Deserpentinization of the peridotite by within a few centimetres of the host rock. the gabbroic magma could promote crystallization and differentiation of the magma because the large enthalpy necessary to dehydrate the serpen- INTERPRETATION tine would help to cool the magma. Thermal decomposition of serpentine is suggested by the association of talc and olivine prisms in some serpen- Relation Between Peridotite and Gabbroic Rock tinite veins (Fig. 4). Similar associations of serpentine, olivine, and talc occur in other peridotites deserpentinized by contact metamorphism Gabbroic magma repeatedly intruded peridotite country rock, incor- (Evans and Trommsdorff, 1970; Trommsdorff and Evans, 1972; Frost, porating blocks of the peridotite in a mesostasis of sheets and dikes. The 1975) and in other deserpentinized peridotites (Vance and Dungan, 1977). inclusions of peridotite are as much as several metres long and are sepa- Deserpentinization is also suggested by the occurrence of cores of serpen- rated by centimetre to metre lenses of gabbroic rock. In other gabbroic tinite, within serpentinized peridotite, in some of the large inclusions hav- bodies in the Trinity complex, Goulland (1977) found the contacts be- ing monomineralic zonations. The water-rich vapor released by the deser- tween the gabbro and peridotite to be everywhere complex and character- pentinization of the peridotite blocks may be absorbed by the gabbroic ized by inclusions of peridotite in the gabbro. Because of the structural, magma, if it is undersaturated with water. A water concentration gradient textural, and mineral-compositional compatibilities, Throckmorton (1978) (more water near the cool block) would suppress the degree of supercool- considered the intrusive gabbro (our pegmatitic gabbros) to be derived ing of the magma adjacent to the inclusion because H2O lowers the from, and roughly coeval with, the massive and layered gabbros and the equilibrium temperature of crystallization. This might explain the absence peridotite. We think that the eastern peridotite (cumulus ultramafic rock of of a fine-grained border of gabbro and the presence of 5-cm-long radially Throckmorton) is an unrelated host rock into which the gabbroic body oriented amphibole crystals about some of the ultramafic blocks. The large intruded because blocks of deformed peridotite occur within the layered crystals (and large intersticies between the crystals) could plausibly in- pyroxenite/gabbro as well as within the pegmatitic gabbros. These blocks crease the magmatic permeability and promote effective migration and were probably deformed before they were incorporated into the gabbroic segregation of residual, differentiated melts. We have no evidence, how- rocks because the orientations of the seams of serpentine in nearby blocks ever, that the eastern peridotite was more serpentinized than was the varies, and deformation is generally not evident in the surrounding gab- western peridotite at the time of intrusion.

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Monomineralic Reaction Zones in Peridotite Inclusions Grain-Size Variations

The monomineralic zones surrounding the serpentinized peridotite The association of igneous hornblende with gabbroic , pla- inclusions probably obtained water from a hydrothermal system contem- gioclase porphyry, and other fine-grained gabbroic rocks is analogous to poraneous with the intrusion of the gabbroic magma. Two plausible the association of granitic pegmatite and with subsolvus sources of water for the zonation of the peridotite inclusions are (Jahns and Tuttle, 1963). We think that the marked grain-size variations deserpentinization of the inclusion or wall rock or an active hydrothermal in the pegmatitic rocks can be so interpreted, and the hornblende provides system. A spherical block of serpentinite could yield enough water to form evidence for a considerable quantity of H2O dissolved in the melt. Local a reaction shell of tremolite approximately two-thirds as thick as the block saturation of the magma with gas promotes migration of the constituents radius. Some of the reaction shells are thicker than this and contain miner- necessary for the growth of large crystals, and episodic loss of gas during als which have more water than does tremolite. The presence of serpentine decompressive events results in supercooling, rapid nucleation, and in the blocks indicates that either extra water was added later to reserpen- millimetre-sized crystals. Episodic and local saturation of the melt with gas tinize the blocks or they did not totally deserpentinize. Consequently, it in the Castle Lake body could result from occasional interaction between seems unlikely that serpentine in the individual blocks was the principal the magma and the intruded serpentinite as well as from crystallization and source of water in their reaction shells. The correlation between reaction decompression. zones and proportion of hornblende in the enclosing gabbroic rock (Table 2) suggests that the reactions occurred during the initial cooling of Pressure of Emplacement of Gabbroic Rocks the magma. Chernosky (1969) found very similar monomineralic reaction zones in ultramafic inclusions in hornblende diorite. We suggest that a Four features of the Castle Lake gabbro which may bear on the hydrothermal source supplied the water for the monomineralic reaction pressure of emplacement are (1) the presence of igneous hornblende in the zones and the hornblende in the gabbro. There is a common association of pegmatitic hornblende gabbro and in the hornblende diorite, (2) the pres- hydrothermal systems with shallow intrusions (Taylor, 1977). The greater ence of vugs in some of the pegmatitic hornblende gabbros and hornblende extent of serpentinization of the peridotites near the gabbroic bodies in the , (3) the patchy metamorphism, and (4) the presence of garnet Klamaths (Hotz, 1971; Goulland, 1977; Quick, 1981) suggests to us that in the hornblende diorite. hydrothermal systems developed during and as a result of intrusion of the Although availability of alkalis (Helz, 1979) and other factors (An- gabbros. derson, 1980) may also be limiting, the andesitic to dacitic melts from which the hornblende crystallized must have had enough water to lower Hydrothermal Metamorphism the crystallization temperature into the field of hornblende stability. The anhydrous liquidus temperatures for basaltic and are The patchy, fracture-related distribution of metamorphism is sugges- between -1210 and 1160 °C (Eggler, 1972; Sekine and others, 1979), tive of retrograde hydrothermal recrystallization. The metamorphism is whereas hornblende is stable in such liquids below -960 °C (Eggler, 1972; compatible with that observed in oceanic gabbros (Miyashiro and others, Allen and Boettcher, 1978). The requisite lowering of the melting tempera- 1979; Ito and Anderson, 1983) but is generally more pervasive and less ture is thus -250 Celsius degrees for and 200 Celsius degrees for sodic (see Mottl, 1983, for review). Metamorphism of oceanic crust by . Using Burnham's (1979) model of hydrous melting of albitic melt sea water produces mostly greenstones with albitic plagioclase either as a (which is suitable for andesitic and dacitic melts), lowering the melting result of addition of sodium or as a result of conversion of anorthite temperature by 200 to 250 Celsius degrees requires 3.7 to 5.4 wt% water; component to epidote or as a result of both. Altered rocks, poor in albite, this corresponds to pressures of -800 to 1,700 bars. Similarly, Spulber and form if the water-to-rock ratio is larger than -20 or if the temperature is Rutherford (1983) found that amphibole coexists with silicic partial melt lower than -300 °C (Mottl, 1983). In view of the incomplete metamor- at pressures greater than -2 kbar in partially melted, hydrous (a less phism, the lack of quartz and the paucity of chlorite, the water-to-rock silicic bulk composition). Although igneous hornblende is stable to great ratio was likely much less than 20. The abundance of actinolite suggests pressures (Allen and Boettcher, 1978), the restriction of igneous horn- temperatures generally above -300 °C. The absence of albitic plagioclase blende to the upper 2 to 3 km in ophiolites (Hopson and Frano, 1977; in even the most differentiated rocks suggests that the metamorphism Stern and others, 1976; Pallister and Hopson, 1981) suggests that a low involved loss of sodium, perhaps in a sodium-poor fluid. Such fluid could pressure of formation is common. be meteoric or distilled from sea water by boiling at pressures <800 bars The comb-layered aspect of fine-grained plagioclase porphyry and (Sourirajan and Kennedy, 1962). associated pegmatitic gabbros is consistent with decompressive (subvol- canic) quenching. Certain other comb-layered rocks appear to have been Sequence of Crystallization emplaced at pressures of 2-3 kbar (Walawender, 1976). The sequence of crystallization in the gabbroic rocks, deduced from Open voids, encased by multiple crystals, in some of the hornblende- the textural and structural relations, is clinopyroxene, plagioclase, horn- bearing gabbroic rocks suggest the crystallization of these rocks at low blende, and quartz. Locally, orthopyroxene is early relative to plagioclase, pressures. The voids in the Castle Lake rocks are larger than are the and olivine precedes clinopyroxene (Throckmorton, 1978). From the adjacent minerals and thus are dissimilar to the small voids present within graphic texture of the pegmatitic hornblende gabbro and from the skeletal certain rapidly decompressed, high-pressure igneous minerals (Roedder, shape in the hornblende diorite, we conclude that igneous hornblende 1965; Andersen and others, 1984). Voids between crystals in plutonic crystallized only from differentiated melts (roughly basaltic andesite for igneous rocks survive only at low pressure because conductive cooling pegmatitic hornblende gabbro whole rock and dacite for hornblende dio- (slower than for volcanic phenocrysts by a factor of at least 10,000) allows rite groundmass). The abundant ilmenite and apatite in these hornblende- time for propagation of grain-boundary cracks and deformation to close bearing rocks also indicates crystallization from differentiated melts (with the voids. Miarolytic cavities filled with fibrous amphibole are present high concentrations of TiC>2 and P2O5). within the hornblende-bearing massive gabbros of the Point Sal

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(Hopson and Franc, 1977), and clay- and carbonate-filled vugs are present Boudier, F., and Coleman, R. G., 1981, Cross section through the peridotite in the Samail ophiolite, southeastern Oman Mountain: Journal of Geophysical Research, v. 86, p. 2573-2592. in hornblende-bearing gabbros and in the Mid-Cayman Rise (Ito Boudier, F., and Nicholas, A., 1977, Structural controls on in the Lanzo peridotites, in Magma genesis: Oregon Department of Geology and Mineral Industries Bulletin 86, p. 63-78. and Anderson, 1983). Such gabbros probably form at depths of 5 km or Bumham, C. W., 1979, The importance of volatile constituents, in Yoder, H. S., ed., The evolution of igneous rocks: less below the rock-water interface at a total pressure of <2 kbar. By Princeton, New Jersey, Princeton University Press, p. 439-482. Chernosky, J. V., 1969, Metasomatic zoning at Tamarick Lake, Trinity County, California [M.A. thesis]: Madison, analogy, voids in the Castle Lake gabbros suggest pressures of crystalliza- Wisconsin, University of Wisconsin, 60 p. Coleman, R. G., 1977, Ophiolites, ancient oceanic ?: New York, Springer, 229 p. tion less than ~2 kbar. Dick, H.J.B., 1977, Partial melting in the Josephine peridotite, I. The effect on mineral composition and its consequence for geobarometry and geothermometry: American Journal of Science, v. 277, p. 801-832. Patchy, fracture-related hydrothermal metamorphism occurs in ophi- Eggler, D. H., 1972, Water-saturated and undersaturated melting relations in Paricutin andesite and an estimate of water olites generally just below the sheeted dikes (Stern and others, 1976; content in a natural magma: Contributions to Mineralogy and Petrology, v. 34, p. 261-271. Evans, B. W., and Trommsdorff, V., 1970, Regional metamorphism of ultramafic rocks in the central Alps: Paragencsis in

Gregory and Taylor, 1981; Ito and Anderse n, 1983) at depths of less than the system Ca0-Mg0-Si02-H20: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 50, p. 481-492. -3 km (~1 kbar). Similar patchy hydrothermal metamorphism is com- Frost, B. R., 1975, Contact metamorphism of serpentinite, chloritic blackwall and at Paddy-Go-Easy Pass, mon around shallow stocks where permeability is partly fracture con- central Cascades, Washington: Journal of Petrology, v. 16, p. 272-313. Goulland, L., 1977, Structure and petrology in the Trinity mafic-ultramafic complex, Klamath Mountains, northern trolled. By comparison, the patchy hydrothermal metamorphism of the California, in Lindsley-Griffin, N., and others, eds., Guidebook to the geology of the Klamath Mountains, northern California: Geological Society of America (Cordiileran Section), p. 112-133. Castle Lake gabbrc suggests a pressure of intrusion of less than ~2 kbar. Gregory, R. T., and Taylor, H. P., 1981, An oxygen profile in a section of Cretaceous oceanic crust, Samail The garnet in the hornblende diorite is likely to be rich in almandine ophiolite, Oman: Evidence of 6lsO buffering of 'xeans by deep (>5 km) sea water-hydrothermal circulation of mid-ocean ridges: Journal of Geophysical Research, v. 86, p. 2737-2755. and perhaps spessartine components because of its association with Heiz, R. T., 1979, Alkali exchange between hornblende and melt: A temperature sensitive reaction: American Mineralo- gist, v. 64, p. 953-965. abundant magnetite, ilmenite, and sulfides, which indicate a bulk composi- Hopson, C. A., and Frano, C. J., 1977, Igneous history of the Point Sal Ophiolite, southern California, in Coleman, R. G., tion with a high Fe+2/Mg ratio and reducing conditions. Both factors are and Irwin, W. P., eds.. North American ophiolites: State of Oregon Department of Geology and Mineral Industries, v. 95, p. 161-183. favorable for the formation of almandine (Hsu, 1968) at low pressures. Hotz, P. E., 1971, Plutonic rocks of the Klamath Mountains, California and Oregon: U.S. Geological Survey Professional Paper, v. 684-B, p. 1-20. Almandine is probably stable below 1 kbar at temperatures as high as 800 Hsu, L. C., 1968, Selected phase relationships in the system Al-Mn-Fe-Si-O, a model for gamet equilibria: Journal of °C (Hsu, 1968) and is found in vugs in volcanic rocks (Miyashiro, 1955). Petrology, v. 9, p. 40-83. Irwin, W. P., 1960, Geologic reconnaissance of the northern Coast Ranges and Klamath Mountains, California, with a Garnet in the hornblende diorite is consistent with a pressure of formation summary of the mineral resources: California Division of Mines Bulletin, v. 179, 80 p. 1981, Tectonic accretion of the Klamath Momtains, in Emst, W. G., ed., The geotectonic development of as low as -1 kbar. California: Englewood Cliffs, New Jersey, Prentice-Hall, p. 29-50. Irwin, W. P., and Upman, P. W„ 1962, A regional ultramafic sheet in eastern Klamath Mountains, California: U.S. Geological Survey Professional Paper, v. 450-C, p. 18-21. Environment of Emplacement Ito, E., and Anderson, A. T., Jr., 1983, Submarine meuimorphism of gabbros from the Mid-Cayman Rise: Petrographic and mineralogic constraints on hydrothermal processes at slow-spreading ridges: Contributions to Mineralogy and Petrology, v. 82, p. 371-388. Jahns, R. A., and Tuttle, O. F., 1963, Layered pegmatite-aplite intrusives: Mineralogical Society of America Special Paper We conclude that the which formed the Castle Lake l,p. 78-92. gabbroic rocks intruded a body of peridotite at low pressure, ~2 kbar or Lanphere, M. A., Irwin, W. P., and Hotz, P. E., 196S, Isotopic ages of the Nevadan and older plutonic and metamorphic events in the Klamath Mountains, Calif.: Geological Society of America Bulletin, v. 79, less. The peridotite; and the gabbroic rocks and magmas were affected by p. 1027-1052. Lipman, P. W., 1964, Structure and origin of an ultramafic pluton in the Klamath Mountains, California: American direct or indirect interaction with a hydrothermal fluid which yielded Journal of Science, v. 262, p. 199-222. widespread but patchy metamorphism characterized by the absence of Loney, R. A., and Himmelberg, G. R., 1977, Geology ol the gabbroic complex along the northern border of the Josephine peridotite, Vulcan Peak area, southwestern Oregan: U.S. Geological Survey Journal of Research, v. 5, p. 761-781. sodic plagioclase. The low pressure of intrusion and absence of sodic Mattison, J. M., and Hopson, C, A., 1972, Paleozoic ages of rocks from ophiolitic complexes in Washington and northern Oregon [abst.]: EOS (American Geophysical Union), v. 53, p. 543. plagioclase are consistent with crystallization in a continental setting as Miyashiro, A., 1955, Pyralspite in volcanic rocks: Geological Society of Japan Journal, v. 61, p. 463-470. argued by Quick (Í981) for the Trinity ultramafic body. Alternatively, it is Miyashiro, A., Shido, F., and Kanehira, K., 1979, Meuisomatic chloritization of gabbros in the Mid-Atlantic ridge near 30°N: Marine Geology, v. 31, p. M47-M52. possible that the Castle Lake gabbroic rocks crystallized in a shallow, Moores, E. M., and Vine, F. J., 1971, The Troodos Miuisif, Cyprus, and other ophiolites as oceanic crust: Evaluation and implications: Royal Society of London Philosophical Transactions, v. A268, p. 443-466. oceanic body of serpentinizing peridotite a nd were metamorphosed by a Mottl, M. J., 1983, Metabasalts, axial hot springs, and the structure of hydrothermal systems at mid-ocean ridges: sodium-poor hydrothermal fluid (yielding greenstone poor in sodic plagio- Geological Society of America Bulletin, v. 94, p. 161-181. Pallister, J. S., and Hopson, C. A., 1981, Samail ophiolite plutonic suite: Field relations, phase variation, cryptic variation clase) derived either from deserpentinization of peridotite or by natural and layering, and a model of a spreading ridge : Journal of Geophysical Research, v. 86, p. 2593-2644. boiling (distillation) of sea water at a pressure <800 bars. Quick, J. E., 1981, Petrology and pedogenesis of the Trinity peridotite, an diapir in the eastern Klamath Mountains, northern California: Journal of Geophysical Research, v. 86, p. 11837-11863.

Roedder, E., 1965, Liquid C02 inclusions in olivine-bcaring nodules and phenocrysts from : American Mineralo- ACKNOWLEDGMENTS gist, v. 50, p. 1746-1782. Sekine, T., Katsura, T., and Aramaki, S., 1979, Water saturated phase relations in some andesites with application to the estimation of the initial temperature and water pressure at the time of eruption: Geochimica et Cosmochimica Acta, v. 43, p. 1367-1376.

Field work was supported in part by National Science Foundation Sourirajan, C., and Kennedy, G. C., 1962, The systen H20-NaCl at elevated temperatures and pressures: American Journal of Science, v, 260, p. 115-141. Grants 35074, 76 -16016, and 79-26485 and the Obering Fund from the Spulber, S. D., and Rutherford, M. J., 1983, The origin of and plagiogranite in oceanic crust: An experimental Department of Geophysical Sciences of the University of Chicago. A. T. study: Journal of Petrology, v. 24, p. 1-25. Stern, C., de Wit, M. J., and Lawrence, J. R., 1976, Igneous and metamorphic processes associated with the formation of Anderson is grateful to M. Garcia, E. M. Moores, N. Lindsey-Griffin, and Chilean ophiolites and their implications for ocxan floor metamorphism, seismic layering and magnetism: Journal G. Himmelberg for helpful guidance. We thank J. Chernosky, C. Hopson, of Geophysical Research, v. 81, p. 4370-4380. Taylor, H. P., Jr., 1977, Water/rock interactions and the origin of H20 in granitic batholiths: Geological Society of D. Jenkins, A. Jones, R. Newton, and H. Wang for helpful discussions and London Journal, v. 133, p. 509-558. Throckmorton, M. L., 1978, Petrology of the Castle Lake peridotite-gabbro mass, eastern Klamath Mountains, California W. P. Irwin, E. Ito, R. Newton, and J. Quick for reviews of earlier [M.S. thesis]: Santa Barbara, California, University of California, 109 p. Trommsdorff, V., and Evans, B. W., 1972, Progressive metamorphism of antigorite schist in the Bergell aureole versions of the manuscript. (Italy): American Journal of Science, v. 272, p. 423-437. Vance, J. A., and Dungan, M. A., 1977, Formation of peridotites by deserpentinization in the Darrington and Sulton areas, Cascade Mountains, Washington: Geolofpcal Society of America Bulletin, v. 88, p. 1497-1508. REFERENCES CITED Walawender, M. J., 1976, Petrology and emplacement of the Los Pinos pluton, southern California: Canadian Journal of Sciences, v. 13, p. 1288-1300.

Allen, J. C., and Boettcher, A. L., 1978, Amphibole in andesite and basalt: II. Stability as a function of P-T-fH20-f02: American Mineralogist, v. 63, p. 1074-1087. Andersen, T., O'Reilly, S. V., and Griffin, W. L., 1984, The trapped fluid phase in the upper mantle xenoliths from Victoria, Austrialia: Implications for mantle : Contributions to Mineralogy and Petrology, v. 88, p. 72-85. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 11,1983 Anderson, A. T., 1980, Significance of hornblende in calc-alkaline sndesites and basalts: American Mineralogist, v. 65, REVISED MANUSCRIPT RECEIVED OCTOBER 16,1986 p. 837-851. MANUSCRIPT ACCEPTED OCTOBER 28,1986

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