Structure and Petrology of the San Gabriel Anorthosite-Syenite Body, Los Angeles County, California
<p>The San Gabriel anorthosite-syenite body is part of large layered intrusive, part of which underlies about 250 square kilometers in the western San Gabriel Mountains between the San Gabriel and San Andreas fault zones 30 kilometers north of Los Angeles. Although not subjected to post-emplac...
| Summary: | <p>The San Gabriel anorthosite-syenite body is part of large layered intrusive, part of which underlies about 250 square kilometers in the western San Gabriel Mountains between the San Gabriel and San Andreas fault zones 30 kilometers north of Los Angeles. Although not subjected to post-emplacement regional metamorphism, the Precambrian anorthosite is intruded by the early Triassic Mt. Lowe granodiorite and the late Cretaceous Mt. Josephine granodiorite, and is deformed by broad folds of at least two ages (Triassic(?) and mid-Cenozoic), which have produced several kilometers of structural relief within the body.</p>
<p>Several sets of faults in the area were active in Tertiary time, but none of them show evidence of Holocene activity. From oldest to youngest, these faults include: (1) several more or less east-west faults with major apparent right-lateral or left-lateral and dip-slip displacements; (2) a NE-trending fault set with important left-lateral displacements; and (3) younger NW-trending faults with small dip-slip and right-lateral displacements. The San Gabriel fault, which lies 1-4 kilometers southwest of its margin, must cut the anorthosite-syenite body at depth and may offset anorthositic rocks at least 28-46 kilometers in a right-lateral sense.</p>
<p>The San Gabriel anorthosite-syenite body is part of a large allochthonous sheet which is floored by a zone of unusual cataclastic gneisses, exposed in the northeastern part of the area in and near to Mill Canyon. Slickensides, lineations and minor folds within this zone suggest latest movement parallel to a N-S or NE-SW direction. Strongly deformed and mylonitized gneisses below the anorthosite include lineated granodioritic gneiss possibly equivalent to the late-Cretaceous Mt. Josephine granodiorite, gabbroic to anorthositic gneisses, and layered amphibolitic gneisses unlike any others seen in the western San Gabriel Mountains. Large thrust displacements probably occurred within this zone as well as along the Vincent thrust 45 kilometers to the east, which resembles this zone and may be related to it.</p>
<p>The anorthosite-syenite body was intruded into previously metamorphosed, granulite-grade Mendenhall gneiss about 1200 million years ago. The body is a large, layered intrusive in which bottom crystal accumulation produced the observed anorthosite-gabbro-syenite differentiation suite. The part of this body now exposed was at least 10 kilometers in thickness and about 15 kilometers in diameter. It probably had the form of an inverted cone, with a sub-horizontal, concordant upper contact. Primary quartz is rare in rocks of this suite; the following lithologies have been distinguished on the basis of the percentage and composition of their constituent feldspars: anorthosite, leucogabbro, gabbro, ferrogabbro (all with calcic andesine), ultramafite (olivine, augite, ilmenite and apatite), jotunite (predominantly antiperthitic sodic andesine), mangerite (antiperthite and mesoperthite) and syenite (predominantly mesoperthite).</p>
<p>The San Gabriel anorthosite-syenite body has been subdivided into three main stratigraphic units (from oldest to youngest): (1) The lowest, largest unit consists of thick sequences of massive anorthosite alternating with layered leucogabbro. This unit is at least 7 kilometers in thickness and becomes more mafic near its top. (2) The overlying syenite unit locally attains thickness of at least 3-5 kilometers, but is commonly much thinner or absent. In some areas, the basal 100-1000 meters of this unit is extremely mafic, but otherwise it is fairly homogeneous and massive, with no cyclic or cryptic layering. (3) The uppermost jotunite unit is a highly compositionally variable unit which intruded overlying granulite gneiss and has been subdivided into 5 subunits. This unit is at least 3-4 kilometers in maximum thickness. It is apparently younger than the syenite, and in places grades downward into syenite. Several masses of hornblende-bytownite gabbro within anorthosite are probably not directly related to the anorthosite-syenite body.</p>
<p>Primary cumulate structures and textures in these rocks have greatly aided the structural interpretation of this body and provide strong evidence of its origin by bottom crystal accumulation. Large, 6 to 25-centimeter hypersthene crystals, which ophitically enclose numerous well-oriented 1 to 3-centimeter plagioclase tablets, indicate a cumulate origin of the leucogabbro. Occasional crescumulate layers in leucogabbro, especially near the margin of the body, formed when first plagioclase and then hypersthene grew from the floor upward into the magma. Slump structures are poorly defined in leucogabbro, but are excellently developed in the mafic lower part of the syenite unit and in some parts of the jotunite unit, and include 1 to 20-meter slump blocks and deformed compositional layers. Many 3-centimeter to 3-meter layers in the mafic lower part of the syenite unit and in the jotunite unit are both size- and density-graded, with coarser, ferromagnesian-mineral-enriched bases, and are extremely useful structural indicators.</p>
<p>Large angular blocks of anorthosite (to 20 meters) are abundant in layered mafic rocks at the base of the syenite unit and in some parts of the jotunite unit. These are slump blocks, which indicate that parts of the syenite and jotunite units accumulated at the base of major tectonic scarps which developed in rocks of the anorthosite-leucogabbro unit which formed the floor of the chambers of the later magmas.</p>
<p>All of the 20 chemical analyses of rocks from this body are iron-rich; the lowest FeO+Fe₂O₃/FeO+Fe₂O₃+MgO ratio (wt. %) is 0.61 and most are between 0.70 and 0.95, which suggest that the original magma was probably similarly iron-rich.</p>
<p>Chemical analyses of these rocks define general trends with substantial scatter on variation diagrams, and in detail those from each of the three units define individual fields with little or no overlap. Thus a common line of descent for all rocks of the body seems somewhat unlikely. The estimated makeup of the entire body is about 46% anorthosite, 23% leucogabbro, 4% gabbro, 12% syenite, 11% jotunite and 4% ultramafite, giving the following suggested average composition: SiO₂, 53.73%, TiO₂, 1.17%, Al₂O₃, 22.33%, Fe₂O₃, 1.66%, FeO, 4.54%, MgO, 1.72%, CaO, 8.08%, Na₂O, 5.05%, K₂, 1.18%, P₂O₅, 0.42%.</p>
<p>Limited mineral composition data show that: (1) Plagioclase in anorthosite and leucogabbro ranges between about An₃₅ and An₅₅ with most between An₄₀ and An₅₀. Plagioclase in not concentrically zoned, but is inhomogeneous with a range of about 3-4% anorthite in individual crystals. (2) There are cyclic compositional variations of plagioclase in the anorthosite-leucogabbro unit, with more albite-rich and more anorthite-rich compositons alternating over hundreds of meters of the stratigraphic section. (3) There is no apparent consistent cryptic variation of proxene and olivine compositions within the syenite and jotunite units.</p>
<p>Postcumulous recrystallization has drastically altered the fabric of most anorthositic and leucogabbroic rocks and has produced extremely coarse grained textures. Pervasive deuteric uralitic alteration of the primary ferromagnesian minerals in all but a few rocks of the syenite and jotunite units suggests that the magma probably had a relatively high water content.</p>
<p>It is possible that all of the rocks of the San Gabriel anorthosite-syenite body could have been produced by differentiation by fractional crystallization of a trachyandesitic parent magma, successive fractions of which were intruded into the magma chamber. However, the detailed sequence of lithologies, mineral compositions and the compositions of the three units suggest that at least two independently generated magmas may have combined to produce the San Gabriel anorthosite-syenite body.</p>
<p>Some important contributions of this study are: (1) the detailed geologic map of the anorthosite-syenite body and determination of its post-emplacement structural history; (2) the description of the complete suite of lithologies and their contact relationships; (3) the description of the mineralogical compositions of each lithology and the recognition of cryptic variation of plagioclase in the anorthosite-leucogabbro unit; (4) the description of the sequence of post-accumulation processes including recrystallization and the hydration of most primary ferromagnesian minerals; (5) the recognition of uninverted pigeonite in rocks of the jotunite and syenite units; and (6) recognition that this andesine anorthosite massif is actually a large deformed stratiform pluton.</p> |
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