The Structure and Petrology of the San Jose Pluton, Northern Baja California, Mexico

Citation

Murray, Jay Dennis (1978) The Structure and Petrology of the San Jose Pluton, Northern Baja California, Mexico. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jrav-6990. https://resolver.caltech.edu/CaltechTHESIS:03282010-104553997

Abstract

The San Jose pluton, a 117-km^2 body of medium-grained biotite-hornblende tonalite in the foothills of the Sierra San Pedro Mártir, northern Baja California, Mexico, is one of the more westerly plutons in the Cretaceous Peninsular Ranges batholith. The pluton has a teardrop form in plan view, elongate north-south and outlined by commonly well-exposed contacts. To the north the outline is remarkably hemi-circular and concordant to isoclinally folded, highly stretched strata. Traced southward, the form becomes increasingly discordant and tapers irregularly to a point. The wall rocks consist of tuffs, tuffaceous sedimentary rocks, and lesser amounts of marble, volcanic flows, and hypabyssal intrusive rocks of primarily basaltic to andesitic composition. Aptian and Albian fossils place the strata in the lower Cretaceous. Hornblende-hornfels facies assemblages are developed near the pluton contacts, but lower greenschist facies assemblages prevail outside the contact aureole. Detailed field and laboratory studies indicate that the pluton is chemically and mineralogically very uniform. The mapping has revealed, however, three texturally distinguishable lithologies each of significant areal extent and coherent form. (1) An interior core of prismatic hornblende tonalite (PHbT) is characterized by elongate prismatic hornblende, thin platy biotite, and a seriate grain-size distribution. (2) An outer horseshoe-shaped partial shell of stubby hornblende tonalite (SHbT) characterized by equant, equigranular, and anhedral poikilitic hornblende and biotite surrounds the PHbT. Together these units comprise the northern two thirds of the body. (3) A seriate porphyritic tonalite (SPT) phase distinguished from the PHbT by the presence of sparse phenocrystic plagioclase grains 6-20 mm in size forms the discordant southern wedge. These major lithologic types represent two, or possibly three, distinct intrusive pulses. Conclusive evidence of the relative ages of the SHbT and PHbT intrusive units is lacking. However, the relationships of both units to several narrow zones of diorite and inclusion-rich tonalite associated with their mutual contact in the northeast, together with the scattered occurrence throughout the interior PHbT unit of inclusions of wall-rock lithologies similar to those around the northern and eastern margins of the pluton, strongly suggest that the SHbT is the younger mass and was emplaced around the perimeter of a pre-existing PHbT pluton. The SPT is completely gradational, both structurally and petrographically, into the PHbT, but is not exposed in contact with the SHbT. The SPT appears to be the youngest of the three units and may be either a late phase of the PHbT intrusive pulse or a separate pulse intruded prior to complete solidification of the PHbT. The SHbT and PHbT units appear to have been emplaced almost entirely by forceful shouldering aside of the wall rocks. The effects of the penetrative deformation and distension of the wall rocks accompanying intrusion (and enhanced by externally-imposed deformation) are intense around the northern contact. These effects decrease gradually southward, giving way to predominantly brittle deformation within the wallrocks around the southern and particularly the southwestern margins. Emplacement of the SPT involved large-scale stoping as well as lateral displacement of the strata. Altogether, stoping appears to have contributed no more than 5-10% to the exposed area of the pluton. Magmatic flow plus marginal protoclasis produced an unusually regular pattern of northward-convex, arcuate foliation in the northern two thirds of the pluton. Foliation is gneissose and conformable to the contact near the pluton margins and decreases to weak in the interior. The dip of foliation changes systematically from steeply outward around the margins through vertical to inward dipping in the interior. Traced southward into the SPT, foliation becomes faint or unrecognizable, the dip increases gradually again to vertical, and the pattern becomes only vaguely concentric and not obviously related everywhere to the local contacts. There is a suggestion of closure (in plan view) of vertical foliation attitudes around the probable center of latest intrusion in the interior of the SPT. This foliation pattern suggests a funnel-shaped geometry of intrusion of PHbT and SPT magmas rising through relatively narrow conduits and expanding primarily to the north and east at essentially the final level of emplacement. The center of intrusion apparently migrated slightly southward with time from the interior of the PHbT into the center of the SPT. The SHbT was emplaced separately around the margins of the PHbT prior to intrusion of much or all of the SPT. Again expansion was preferentially to the north and east. The pattern of deflection of wall rocks around the pluton also indicates predominantly northward and eastward expansion; lesser southward expansion was limited to late fracture-controlled breaking out of the SPT magma from the concordant confines of the northern masses. Petrographic and structural observations indicate that preferred mineral orientation developed primarily by rotation of crystals in the magma rather than by solid-state deformation and recrystallization. However, flow deformation may have continued locally after the completion of magmatic crystallization, particularly in the protoclastic gneissose rocks. Variations in the intensity of foliation and protoclasis in the tonalite and of penetrative deformation and distension in the wall rocks are attributed in large part to the history of progressive intrusion and asymmetric expansion. However, the pluton was apparently syntectonic, and externally-imposed deformation not directly related to emplacement of the San Jose pluton also contributed to the deformation of the wall rocks and outer parts of the tonalite, particularly around the northern and eastern margins. Systematic lineation patterns developed in both the wall rocks and the marginal gneissose tonalite in the latter areas suggest two sources of externally-imposed deformation: (1) southward expansion or shoving of the nearby Las Cochas pluton towards the northern margin of the San Jose pluton, and (2) deformation directed apparently upward and southwestward from the northeast and east, possibly in response to plutonic emplacement or regional uplift in the Sierra San PedroMartir. Although the oval outline and striking concordance of the pluton may be superficially suggestive of en masse diapiric emplacement, as opposed to gradual rise of magma through narrow channelways and balloon-like expansion at the final level of emplacement, the asymmetry of both the foliation pattern in the tonalite and the deformation of the wall rocks appears inconsistent with such a mechanism. Mineralogically, the tonalite consists on the average of 64% plagioclase, 16% quartz, 11% hornblende, 5-6% biotite, and <0.3% alkali feldspar. Plagioclase, zoned from cores of An_(40-45) to rims of An_(20-30), was the principal liquidus phase, accompanied by minor clino-pyroxene in the SHbT and by hornblende and locally minor clinopyroxene in the PHbT and SPT. Biotite crystallized as a late interstitial or poikilitic phase primarily in coexistence with hornblende rather than as a reaction product. The trace amounts of late interstitial alkali feldspar (sanidine-orthoclase) are unusually potassic (0r_(90-95)) and do not appear to have crystallized in equilibrium with plagioclase; much or all of the potassium feldspar and possibly some of the biotite may have crystallized from a late-stage exsolved vapor phase. The assemblage of opaque minerals -- nearly pure magnetite coexisting with complex lamellar intergrowths of titano-hematite and ferrian-ilmenite --implies crystallization under unusually oxidizing conditions apparently approaching those of the Mn0-Mn_30_4 buffer during much of crystallization, and increasing to those of the hematite-magnetite buffer at near-solidus conditions. Low Fe/(Fe + Mg) ratios in biotite (0.40-0.44), hornblende (0.32-0.36), and relict clinopyroxene (0.23-0.26) and low whole-rock Fe0/(Fe) + Fe_2O_3) ratios (0.408-0.515) also record unusually oxidizing conditions. Textural characteristics distinguishing the SHbT phase, on the one hand, from the PHbT and SPT phases, on the other, appear to be due to slight differences in f_0_2, f_H_2O and possibly temperature throughout all but the late stages of crystallization (SHbT magma drier, less oxidizing, and possibly slightly hotter). Both the modal and chemical data confirm the petrologic significance of the textural units defined in the field. Together the PHbT and SPT define one set of variation trends which, for a given Si0_2 or modal quartz content, are characterized by slightly higher normative or modal plagioclase content and slightly lower normative or modal color index than the combined trends of the SUbT and its gneissose border phase. The SPT samples show a much more restricted range of modal and chemical compositions than do the PHbT samples, supporting the identification of the SPT as a third unit closely related to the PHbT. The composition of the outer gneissose phase of the SHbT is also distinctive, being slightly more felsic rather than more mafic than the rest of the tonalite. Despite the systematic compositional distinctions and the evidence for at least two, possibly three separate pulses of intrusion, both the modal and chemical compositions are remarkably uniform. SiO_2 contents range from 59.4 to 65.2% (exclusive of the gneissose border rocks). Variations in other elements are correspondingly much smaller. The SHbT and SPT, in particular, are both exceptionally homogeneous, with Si0_2 contents consistently in the ranges 60.1-62.0 and 61.8-64.1%, respectively. Yet both the strong compositional zoning in plagioclase and the sequence of mineral crystallization indicate that the composition of the melt changed considerably as crystallization proceeded. The homogeneity of the tonalite, particularly within the individual textural units, therefore implies highly viscous magmas in which processes of segregation or differential migration of crystals relative to melt (e.g., gravity settling or flow differentiation) were generally ineffective except on a small scale (local schlieren). Multiple emplacement of such large volumes of homogeneous magmas, rising essentially through the same conduit, without visible association of materials with other compositions argues strongly that the tonalite magmas were generated as primary magmas. Chemically, the tonalite is characterized by a calcic alkali-lime index (63); high Na_2O and low K_2O contents (average: 4.82 and 0.74%, respectively), hence high Na_2O/K_2O ratios (average: 6.51); high Al_2O_3 and normative plagioclase contents (18.17 and 66.37%, respectively); and low Fe0/(Fe0 + Fe_2O_3) ratios (average: 0.463). The pluton is depleted in Rb, U, Th, and rare earth elements, and the ^(87)Sr/^(86)Sr initial (0.7036) and Pb isotopic ratios (206/204 = 18.56, 207/204 = 15.58, and 208/204 = 38.19) are relatively unradiogenic. The low concentrations of K and other incompatible elements and the relatively unradiogenic Sr initial and Pb isotopic ratios rule out significant contribution from older sialic crust or related sedimentary rocks. Partial melting of a material chemically similar to ocean floor (ocean ridge or abyssal) basalt, whether subducted oceanic crust or basaltic areas in the overlying mantle or lower crust, appears to best satisfy all of the compositional constraints.

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