Tectonic Emplacement of the Burro Mountain Ultramafic Body, Santa Lucia Range, California

Burch, Stephen H.

doi:10.1130/0016-7606(1968)79[527:teotbm]2.0.co;2

Cited by 25 publications

(12 citation statements)

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“…Hess (1960) cited occurrences in the Stillwater ultramafic zone; Wagner (1929) and Cameron and Desborough (1964) described examples in the Bushveld complex; and Burch (1969) discussed occurrences in alpine ultramafic complexes. In most cases, the main minefral replaced by olivine is orthopyroxene, and the interpretations offered are based on the Bowen and Tuttle (1949) experimental observation that enstatite can be converted to forsterite by leaching and diffusion of silica through an aqueous vapor phase.…”

Section: Formation Of the Replacement Dunite Peridotite And Assomentioning

confidence: 99%

Petrology of the Duke Island Ultramafic Complex Southeastern Alaska

Irvine

1974

Geological Society of America Memoirs

231

260

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Duke Island, with an area of about 60 sq. mi., is situated on the North American Pacific Coast at latitude 55° N. The ultramafic complex exposed on the island is the southernmost of some thirty distinctive ultramafic bodies of Early Cretaceous age distributed along the 350-mi. length of southeastern Alaska. A similar belt of corresponding bodies occurs in the Ural Mountain; there are at least two complexes of the same type in British Columbia, a possible occurrence in California, and two in Venezuela.The oldest rocks on Duke Island are clastic sedimentary and volcanic rocks of probable Late Triassic or Jurassic age. These are metamorphosed to greenschist-and amphibolite-facies grades and intruded by gabbroic and granitic rocks. The ultramafic rocks underlie two main areas totalling 9 sq. mi., and about a dozen minor areas. They are emplaced in gabbro that is intensely amphibolitized 1 around their edges and permeated by highly aluminous hornblende-anorthite (An 90 96 ) pegmatite derived from the ultramafic complex.The unaltered gabbroic rocks have tholeiitic affinities and are largely cumulates. They comprise two differentiation series, one ranging from picrite through olivine gabbro to hypersthene gabbro, and the other ranging from norite to olivine ferronorite. Olivine has reacted to orthopyroxene in both series, and hornblende is a late phase in all rock types. Some of the hornblende appears to be primary (postcumulus), but most is metamorphic or metasomatic in origin.The main ultramafic rock types, in order of abundance, are olivine clinopyroxenite, hornblende-magnetite clinopyroxenite, peridotite (wehrlite), dunite, and hornblendite. In many places the olivine-bearing units show well-developed rhythmic layering featuring pronounced grain-size sorting and remarkable sedimentation structures. It is evident, therefore, that the rocks are mainly cumulates, and that the layers were deposited by magmatic currents, which at times must have closely resembled turbidity currents. Hornblende-magnetite clinopyroxenite shows similar layering at one place, with a few of the layers being graded in modal content of magnetite. Another kind of layering, termed "inch-scale layering," common to olivine clinopyroxenite, is defined by vague alternations of granular olivine and pyroxene plus more massive units of perpendicularly oriented pegmatitic pyroxene and small discontinuous bands of dunite, all with 'The term amphibolitization, as used throughout this book, refers to the process in which a rock (at Duke Island, gabbro) is changed into amphibolite. 1 on June 25, 2015 memoirs.gsapubs.org Downloaded from on June 25, 2015 memoirs.gsapubs.org Downloaded from ABSTRACT 3crystallization of postcumulus hornblende. The peridotite is a combination of the first two stages generated (in effect) by mixing in the magmatic current system. The intense recrystallization of the olivine clinopyroxene cumulates and their replacement by the dunite are believed to be due to transfer of materials via an aqueous vapor phase filtering through the cu...

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Section: Formation Of the Replacement Dunite Peridotite And Assomentioning

confidence: 99%

Petrology of the Duke Island Ultramafic Complex Southeastern Alaska

Irvine

1974

Geological Society of America Memoirs

231

260

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“…On the basis of a strong correlation between ultramafic rocks and aeromagnetic anomalies in the northern Coast Ranges and Klamath Mountains, Irwin and Bath (1962) inferred that ultramafic rocks are the source of the Great Valley magnetic anomalies. A serious difficulty for this explanation lies in the observations (Burch, 1965;Saad, 1969) that dense, unserpentinized ultramafic rocks are relatively nonmagnetic. When they become strongly serpentinized and magnetic, they no longer have a high density.…”

Section: Controversy Over the Source Of The Great Valley Anomaliesmentioning

confidence: 99%

Magnetic and Gravity Anomalies in the Great Valley and Western Sierra Nevada Metamorphic Belt, California

Cady¹

1975

Geological Society of America Special Papers

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Magnetic and gravity anomalies, with typical amplitudes of 1,000 7 and 50 mgal, respectively, occur over the Great Valley of California. Aeromagnetic data were combined to make a composite aeromagnetic map of most of the Great Valley and adjoining areas. Good correlation between local details of the gravity and magnetic anomalies suggests that dense, magnetic rock is the source of the anomalies. A nearly continuous magnetic high and weak gravity highs occur where serpentinite is exposed along the Coast Range thrust fault. Magnetic highs, but no associated gravity highs, occur where serpentinite crops out in the western Sierra Nevada metamorphic belt. The metavolcanic rocks are relatively nonmagnetic. Major magnetic highs with strong associated gravity highs are caused by gabbro at three places in the western Sierra Nevada. Two of these gabbro occurrences are probably part of ophiolite complexes.Gravity, magnetic, and drill-hole data were used to construct a map of basement rock types. Gabbro and similar mafic rocks are abundant beneath the crest of the Great Valley anomalies. Ultramafic rocks are very rare in drill holes that reach basement. A major break in the anomaly patterns suggests a possible east-trending fault in the basement rocks near Fresno.A two-dimensional crustal model was made across central California through the use of seismic refraction data and gravity and magnetic modeling. Magnetic rock comes to within 2.5 km of the surface just east of the center of the Great Valley and dips steeply to the west beneath the western side of the valley. If gabbro is the source of the anomalies, then sialic crust must be virtually nonexistent beneath the Great Valley. Analogy with the ophiolite complexes of the Sierra Nevada foothills suggests that the source of the Great Valley anomalies is a tectonically emplaced fragment of oceanic crust.In Late Jurassic time, an eastward-dipping subduction zone in the western Sierra Nevada foothills became detached and stepped westward to the present position of the Coast Range thrust fault, leaving behind a fragment of oceanic crust. I propose that this fragment, covered by Tithonian and younger strata, causes the Great Valley magnetic and gravity anomalies.

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“…The tight folding may be related not only to the relatively small sedimentary thicknesses, but also to the presence of a mobile layer of serpentinite at shallow depths. Other field studies (for example, Thayer and Guild, 1947;Shagam, 1960;Burch, 1968) have also shown that serpentinite is mobile enough to be emplaced along faults without significant contact metamorphic effects.…”

Section: Diapiric Emplacement Of Serpentinitementioning

confidence: 93%