Prebatholithic Rocks Research Articles

Distinctive Triassic megaporphyritic monzogranite: Evidence for only 160 km offset along the San Andreas Fault, southern California

Distinctive megaporphyritic bodies of monzogranite to quartz monzonite that occur in the Mill Creek region of the San Bernardino Mountains and across the San Andreas fault on Liebre Mountain share identical modal and chemical compositions, intrusive ages, and petrogenesis and similar thermal histories. Both bodies are strontium‐rich and contain large potassium feldspar phenocrysts and hornblende. U‐Pb determinations on zircon from both bodies indicate Triassic intrusive ages (215 Ma) and derivation, in part, from homogeneous Precambrian continental crust. U‐Pb analyses on apatite and sphene and K‐Ar analyses on hornblende and biotite show that the bodies suffered a Late Cretaceous thermal event (70–75 Ma). The strong similarities between the two bodies suggest that they constitute segments of a formerly continuous pluton that has been offset about 160 km by movement on the San Andreas fault, about 80 km less than the generally accepted distance. Plutons having monzonitic compositions, reassembled with the megaporphyritic bodies are used as a piercing point, form a relatively coherent province within the varied suite of Mesozoic batholithic and prebatholithic rocks in southern California.

Kaweah serpentinite mélange, southwest Sierra Nevada foothills, California

Prebatholithic rocks of the southwest Sierra Nevada foothills contain a 125-km-long northwest-trending disrupted and metamorphosed ophiolite belt. Much of the belt consists of a serpentinite-matrix melange in which the ophiolitic material is dispersed. The melange matrix consists of schistose serpentinite derived from tectonitic peridotite and sedimentary serpentinite. The tectonic blocks consist mainly of peridotite, gabbro, basalt, chert, ophicalcite, and silica-carbonate rock and are as long as several kilometres. Within the tectonic blocks relict primary features such as bedding, pillows, dikes, and cumulate layering remain. The melange represents disrupted and internally mixed oceanic lithosphere of latest Paleozoic to possibly earliest Mesozoic age. Outcrop mapping of the melange reveals clustering of blocks into several lithologic associations. The associations are defined as melange units, which appear to represent the vestiges of once-intact ocean-floor sections. Three types of melange units have been recognized: (1) peridotite-gabbro units; (2) gabbro-basalt units; and (3) peridotite-chert-basalt units. The first two units represent crust and upper mantle whose deformation and metamorphic history began at the site of ocean-floor genesis. The third unit represents abnormal crust composed of ultramafic protrusions and interbedded sedimentary serpentinite, ophicalcite, chert, and pillow lava. Protrusive activity also started at the site of ocean-floor genesis and continued for an extended time after sea-floor–spreading transport of the ophiolite belt away from the genesis site. Genesis was at an oceanic spreading center that was cut by a transverse fracture zone. Ocean-floor melange developed along the fracture zone by the combined effect of protrusive acitivity and wrench faulting. Emplacement of the fracture-zone complex (ophiolite belt) resulted from large-scale wrench faulting that truncated the ancient continental margin and juxtaposed the complex against the modified margin. During transport to the continental margin, a chert-argillite oliostostrome complex was shed across the ophiolite belt. The olistostromes carried limestone blocks with fauna exotic to North America. Once in the vicinity of the continental margin, the ophiolite belt served as basement for continent-derived submarine-fan deposits and island-arc volcanic rocks, both of Late Triassic to Middle Jurassic age. Deformation of these strata along with their ophiolitic basement continued along the older fracture zone trends. The strata now exist as highly deformed depositional remnants above serpentinite melange.

Early Mesozoic tectonic evolution of the western Sierra Nevada, California

Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was constructed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and collided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giving birth to the Franciscan-Sierran arc-trench complex. Fault zones and melanges in the western Sierra Nevada reflect the complex suturing at the collision boundary. Pre-Tithonian ophiolite at the base of the Great Valley sequence in the Coast Ranges originated in a back-arc or marginal basin setting with respect to the coeval Sierran foothills arc.

Radiogenic heat production in prebatholithic rocks of the central Sierra Nevada

Likely source material for the Sierra Nevada batholith is represented by exposures of Paleozoic rocks on the western flanks of the Sierra Nevada, Paleozoic sedimentary rocks in roof pendants east of the Sierran crest, and both Precambrian and Paleozoic rocks in the western portion of the Basin and Range province. The radiogenic heat production of these rocks has been calculated from γ-radiometric determinations of their U, Th, and K contents. The abundances of these radioisotopes in carbonate rocks are roughly an order of magnitude lower than those in adjacent siliceous sedimentary rocks; therefore, the radiogenic heat production in the Paleozoics depends mainly on the relative amounts of carbonate and siliceous material. Heat production varies from east to west, with weighted values (expressed in units of μcal/g yr) of 3.7 in Precambrian rocks of the Death Valley area, 3.5 in Paleozoics of the Inyo and White mountains, 3.8 in Paleozoic pendants, and 2.1 in rocks of the Calaveras formation on the western slope. A difference, similar to that in the Paleozoics, is observed between Mesozoic rocks exposed near the crest (4.4) and the Mesozoics of the western foothills (2.4). The geographic distribution of heat production matches (though not proportionally) that in the batholithic rocks. This suggests that there was a regional distribution of radioactivity in likely source material for the granitic rocks that now, strongly accentuated by the effects of erosion, is reflected in the batholith.

Petrogenesis of Mylonites of High Metamorphic Grade in the Peninsular Ranges of Southern California

A fairly continuous, narrow belt of mylonite gneisses extends approximately 60 miles across southern California and crops out prominently at Coyote Mountain, near Borrego Springs, San Diego County. At Coyote Mountain, both prebatholithic rocks and igneous rocks lithologically similar to rocks from the nearby southern California batholith have been deformed in the mylonite zone—a deformation that is the last plutonic event recorded in the rocks. Petrographic evidence within these mylonites at Coyote Mountain indicates that sillimamte-K feldspar-muscovite-quartz assemblages remained stable or recrystallized (or both) during mylonitization. In addition, a maximum “set” temperature of 580° C, inferred from the MgCO 3 content of calcite in deformed dolomite marbles, was determined. The physical conditions probable at the metamorphic peak accompanying mylonitization are: P-T conditions inferred from experimentally studied systems, T = 580° to 660° C and Pxotai = PH, O( ? ) = 3.4 to 7.0 kb; high H 2 O activity, as indicated by the continued stability of muscovite at such high temperatures. Mylonitization within this belt may be related to a rise of magma to higher crustal levels from within the southern California batholith.

Age of Precambrian events in the Northeastern Front Range, Colorado

Precambrian metasedimentary rocks of the northeastern Front Range have undergone a long and complex geologic history involving multiple periods of metamorphism, deformation, and intrusive activity. The first event following sedimentation involved deformation which produced large east-west isoclinal folds and metamorphism in the greenschist facies and possibly higher. It has been impossible to date this first event by radiometric techniques. The second major event was a period of dominantly medium- to high-grade regional metamorphism accompanied by folding and syntectonic intrusions, such as the Boulder Creek granite. Rb-Sr dating of high-grade gneisses, pegmatites, and Boulder Creek granite places this event at 1700–1800 m.y. ago. Precambrian metasedimentary rocks younger than 1700 m.y. have not been recognized in the northeastern Front Range. A major period of plutonism is dated at 1390–1450 m.y. ago during which time the Sherman and Silver Plume granites, pegmatites, and basalt and andesite dikes were emplaced. The regional heating of the country rock at this time was effective in lowering most of the Rb-Sr and K-Ar mineral ages determined on prebatholithic rocks and probably caused the retrogressive metamorphism which locally affected the gneisses and schists. Post-batholith faulting and cataclasis, possibly at about 1300 m.y., may have been effective in further reducing some mineral ages.

STRUCTURE, STRATIGRAPHY, AND PALEONTOLOGY OF AN UPPER TRIASSIC SECTION ON THE WEST COAST OF BRITISH COLUMBIA

The highly deformed section at Open Bay is one of the few good exposures of a thick sedimentary unit within the prebatholithic rocks along coastal British Columbia. It provides new structural information relating to emplacement of a part of the Coast Range batholith and it contains an important Upper Triassic fauna unusually well represented. Structural and paleontological analyses are mutually supporting and are purposely combined in one paper.Thirteen ammonite genera from 14 localities clearly substantiate McLearn's tentative assignment to the Tropites subbullatus zone (Upper Karnian) and suggest a restriction to the T. dilleri subzone as defined in northern California.Contrary to an earlier view, the beds are lithologically similar across the whole bay except for variations in the intensity of deformation and thermal alteration. Their contact with slightly older relatively undeformed flows is apparently a zone of dislocation. Stratigraphic thicknesses cannot be measured with confidence, and subdivision into "Marble Bay Formation" and "Open Bay Group" cannot be accepted. Open Bay Formation is redefined to include all the folded marble and interbedded pillow lava at Open Bay. Lithologic and biostratigraphic correlation is suggested with the lower middle part of the Quatsino Formation on Iron River, 24 miles to the southwest. Basalt flows and pillowed volcanics west of Open Bay are correlated with the Texada Formation within the Karmutsen Group.The predominant folding is shown to precede, accompany, and follow intrusion of numerous andesitic pods and to precede emplacement of quartz diorite of the batholith. Structural asymmetry is shown to have originated through gentle cross-folding and emplacement of minor intrusives during deformation.

Lower Cretaceous Pre-Batholithic Rocks of Northern Baja California, Mexico: GEOLOGICAL NOTES

Lower Cretaceous Pre-Batholithic Rocks of Northern Baja California, Mexico: GEOLOGICAL NOTES

Uranium deposits of the northern part of the Boulder Batholith, Montana

Uranium minerals and radioactivity anomalies occur in many silverlead veins and chalcedony veins and vein zones in the Boulder batholith of southwestern Montana. Pitchblende has been identified in a few silverlead veins. These silver-lead veins occupy shear zones along which there is no evidence of large-scale lateral displacement. The wall rock adjacent to the veins is intensely silicified and sericitized quartz monzonite and granodiorite. The veins have yielded substantial quantities of lead, silver, zinc, and gold. The silver-lead veins consist principally of galena, sphalerite, tetrahedrite, chalcopyrite, and pyrite in a gangue of light- to dark-gray quartz, altered rock, gouge, and subordinate chalcedony and carbonate minerals. No anomalous radioactivity nor uranium minerals have been found in similar veins in prebatholithic rocks of the area.Chalcedony veins and vein zones, some of which are uraniferous, are distinctly different from the silver-lead veins and, with a single exception, are known only in the batholith. The chalcedony vein zones consist of one or more discontinuous stringers or veins of chalcedony and microcrystalline quartz in silicified and sericitized quartz monzonite and granodiorite, and in less strongly altered alaskite. Only small amounts of silver ore have been produced from these chalcedony veins and vein zones. All of the veins are early Tertiary in age, but the silver-lead veins probably are older than the chalcedony veins. Uranium is closely associated with chalcedony and microcrystalline quartz in both types of veins. This association suggests that all of the uranium in the area is of the same age. If so, some of the silver-lead veins must have been re-opened during the period of chalcedony vein formation.

DEFORMATION AND IGNEOUS INTRUSION IN SOUTHERN BRITISH COLUMBIA, CANADA

Regional deformation in southern British Columbia continued from Late Jurassic to Eocene, progressing from Vancouver Island and the Coast Range in the west eastward to the Rocky Mountains. The batholiths, reflecting this progression, are dioritic to granodioritic on Vancouver Island, predominantly granodioritic in the Coast Range, and principally granitic in central and eastern British Columbia. Regional linear elements-trends of formations, folds, and faults-tend to parallel the margins of the major intrusions. The pre-batholithic rocks were deformed approximately at the time of batholithic intrusion. A detailed structural map of southern British Columbia illustrates these features.