Howard W. Day

Paleozoic metamorphism in the Qinling orogen, Tongbai Mountains, central China

The collision of the Sino-Korean and Yangtze blocks to form a significant part of China is recorded in the Qinling, Tongbai, and Dabie Mountains. Radiometric ages of the ultrahigh-pressure metamorphic rocks in the South Qinling orogenic belt suggest that subduction and collision took place during the Triassic Period. Our new 40 Ar/ 39 Ar geochronology of units in the North Qinling orogenic belt confirms that high-grade metamorphism and deformation took place also during the Silurian-Devonian and Carboniferous Periods. These results imply that the amalgamation of eastern China was a multistage process extending over at least 200 m.y.

A revised diamond-graphite transition curve

Abstract The transition from diamond to graphite is a key equilibrium for interpreting ultrahigh-pressure metamorphic rocks. Despite widespread interest, there remain significant systematic differences between the best available experimental determinations of P and T (Kennedy and Kennedy 1976) and numerous thermodynamic calculations of the transition. At temperatures below 1400 K, calculated equilibrium pressures are lower than extrapolations of the experiments by as much as 5 kbar. At 3000 K, calculated pressures vary from more than 8 kbar above to almost 20 kbar below the position of the extrapolated transition. A revised curve based on a critical review of the experimental and thermodynamic data is consistent with expanded experimental brackets and the preferred calorimetric data. It is steeper than the transition proposed by Kennedy and Kennedy (1976) and previous calculations and passes through 16.2 kbar, 298 K; 33.9 kbar, 1000 K; 63.5 kbar, 2000 K; and 98.4 kbar, 3000 K. The revised curve implies that the minimum pressure for formation of diamond-bearing crustal rocks is 3-4 kbar less than implied by extrapolation of the experiments. Because the revised transition is steeper than most previous calculations, the triple point among graphite, diamond, and liquid carbon may be as much as 40 kbar higher than previously estimated.

Structure and tectonics of the northern Sierra Nevada

Recent detailed mapping suggests a new working hypothesis for the structure of the northern Sierra Nevada. We propose that pre-Cretaceous rocks are deformed by a series of eastward-directed over-thrusts modified by west-directed folds and faults. The highest and westernmost tectonic unit is the Jurassic Smartville complex. Structurally below it, there are imbricate thrust slices of Jurassic and older ophiolitic and oceanic sedimentary rocks in the Central belt and of Paleozoic to Jurassic sedimentary and volcanic rocks in the Eastern belt. The Paleozoic Feather River peridotite separates the Eastern and Central belts, and our hypothesis suggests that both of its margins may be important thrust faults. The east-directed faults and folds are deformed by northwest-trending, upright, and west-vergent folds and reverse faults that control the present outcrop pattern. These later structures have so modified the earlier east-directed structures that the latter have been recognized only recently. An important key to understanding the structure has been the recognition of ophiolitic complexes that can be correlated across major faults and that contain a pseudostratigraphy useful in determining local structures. The events recorded by both the east- and west-directed deformations occurred during Callovian through Kimmeridgian time and represent the Nevadan Orogeny. Our hypothesis of early east-directed overthrusts followed by west-directed back folding and faulting implies shortening and thickening of the crust during the Nevadan Orogeny and is consistent with the idea that the northern Sierra Nevada is the result of a crustal collisional process.

The Smartville intrusive complex, Sierra Nevada, California: The core of a rifted volcanic arc

The Smartville complex is a Jurassic volcanic and plutonic arc in the northwestern Sierra Nevada that was deformed during the Late Jurassic Nevadan “orogeny.” We interpret the Smartville intrusive complex to have formed during the incipient rifting of an active volcanic arc. This interpretation is supported by the close relationship between volcanism and plutonism and by the close association between sheeted dikes and the younger plutonic rocks. Further support is found in the similarities that exist between the Smartville complex and modern arcs that developed on oceanic crust, either at a continental margin or in the ocean basins. Smartville volcanic rocks consist of a lower unit of tholeiitic submarine flows and pillowed flows that grades upward into an upper unit of calc-alkaline pyroclastic and volcaniclastic deposits. Intrusive rocks include older units of metamorphosed gabbro and massive diabase, which are intruded by a unit of 100% mafic and felsic, sheeted and unsheeted dikes. Biotite-hornblende tonalite and granophyric hornblende tonalite plutons are coeval with the dike complex, and both rock types occur locally as dikes within the dike unit. Continuously and reversely zoned gabbro-diorite plutons are also coeval with the dike complex. Granodiorite plutons are the youngest intrusive unit and may be related to the Sierra Nevada batholith. Relative age relations suggest that the Smartville complex formed in a single volcanic-plutonic system of pre-Nevadan age. The dike complex intrudes and is intruded by both the tonalite and zoned gabbro-diorite plutons. Both the upper and lower volcanic units are intruded by all of the plutonic and hypabyssal units and were deformed prior to the intrusion of the sheeted dike complex. Clasts of plutonic and hypabyssal rocks resembling the younger intrusives, however, occur locally in some of the youngest volcaniclastic rocks, suggesting that shallow plutonism and volcanism could be broadly coeval. Early Nevadan thrust faults juxtapose volcanic and older plutonic rocks of the Smartville complex and Mesozoic(?) chert-argillite broken formation to the east. The younger plutons and the dike complex in the Smartville are deformed by steep, late Nevadan faults and do not intrude chert-argillite broken formation. The youngest granodiorite plutons in the area truncate Nevadan faults, intrude the chert-argillite formation, and appear to be unrelated to the Smartville complex. The Smartville volcanic arc underwent pre-Nevadan intra-arc extension. The sheeted dike complex is the primary manifestation of the rifting event. The elongate shapes of the younger plutons, which are coeval with the dike complex, reflect extensional control on their emplacement. Volcanic rocks in the western and southern Smartville complex were deformed prior to the intrusion of the dike complex.

Overthrust model for the Sierra Nevada

The structure of the northern Sierra Nevada, California, is a series of east-directed thrust faults modified by upright and west-directed folds and faults. The highest tectonic unit is a nappe composed of the Smartville ophiolitic complex and related rocks. These structures collectively represent the Middle Jurassic Nevadan orogeny. This overthrust-backfold model resembles Middle Jurassic events in British Columbia and the Cretaceous-Tertiary tectonics of the western European Alpine belt, and it is consistent with the hypothesis that the Nevadan orogeny reflects the collision of an oceanic volcanic arc with North America.

Tectonic setting of the Jurassic Smartville and Slate Creek complexes, northern Sierra Nevada, California

Jurassic ophiolitic complexes in California and Oregon are widely distributed outboard of a Jurassic and older continental margin. Whether these outboard complexes were exotic and collided with the Jurassic continental margin, or whether they were simply part of a wide forearc to a Jurassic continental magmatic arc, has long been at issue. We have studied two such complexes and crosscutting intrusions in the fault-bounded Central and Western belts of the northern Sierra Nevada. We report new and revised U-Pb (zircon) dates for the Early Jurassic and younger Slate Creek complex (ca. 203−171 Ma) in the Central belt; the Late Jurassic Smartville complex (159 ± 3 Ma) in the Western belt; and crosscutting intrusions ranging from Middle Jurassic to Early Cretaceous age. The new ages imply that the Central and Western belts have been part of the same terrane since Middle Jurassic time and that the fault separating them has accommodated only minor displacements since ca. 160 Ma. Isotopic compositions of common Pb indicate derivation from mantle sources, but evidence for inheritance of Pre-cambrian zircon is common in plutons within the Early and Late Jurassic complexes and in Middle Jurassic, Late Jurassic, and Early Cretaceous crosscutting intrusions. Although there are several potential sources for such contamination, the ages of the inherited zircon are consistent with a North American origin. This contamination would have been accomplished most plausibly near the North American margin.

Evolution of perthite composition and microstructure during progressive metamorphism of hypersolvus granite, Rhode Island, USA

The Scituate Granite in central Rhode Island, USA contains very coarse alkali feldspar mesoperthite and has been subjected to prograde metamorphism subsequent to original igneous cooling. The modal abundance and grain size of relics of alkali feldspar mesoperthite decrease systematically as metamorphic grade increases. The composition and microstructures of coexisting phases in the relict perthite grains also vary systematically with increasing metamorphic grade. Microstructures coarsen and become more complex, and compositions record increasing metamorphic temperatures.We suggest that the microstructures and compositions have been produced by exsolution, either during post-igneous cooling or early metamorphism, followed by partial homogenization during prograde metamorphism. The principal control on the evolution of the microstructures and compositions was probably the maximum temperature achieved during prograde metamorphism, with the abundance and size of perthite relics determined by recrystallization during deformation.

Timing of arc construction and metamorphism in the Slate Creek Complex, northern Sierra Nevada, California

Late Paleozoic to early Mesozoic rocks in the Sierra Nevada and Klamath Mountains, western United States, preserve a record of lateral growth of continental crust by incorporation of ophiolites, volcanic arcs, and associated sedimentary rocks. Deciphering the timing of arc construction and metamorphism is critical for elucidating tectonic and thermal evolution during this type of crustal growth, but is difficult in complex accretionary terranes. In this study, we address the timing of arc construction and regional metamorphism in the Slate Creek Complex, a volcano-plutonic terrane in the central part of the northern Sierra Nevada. New 40 Ar- 39 Ar ages from relict volcanic hornblende demonstrate that the youngest volcanic unit in the Slate Creek Complex is ca. 170 Ma, at least 30 m.y. younger than previous estimates of ca. 200 Ma for metaplutonic rocks in the lower part of the complex. Consequently, the Slate Creek Complex is polygenetic, with two distinct episodes of arc magmatism. The distribution of mineral assemblages and textures indicates that greenschist and epidote-amphibolite facies metamorphism is younger than 170 Ma and is associated with crosscutting plutons that yield cooling ages of 150 Ma and younger. Amphiboles from foliated rocks in the Slate Creek Complex yield plateau ages of 156.1 ± 0.6 and 152.0 ± 0.7 Ma that date cooling subsequent to or dynamic recrystallization during the metamorphic event. Age and lithologic similarities suggest that the lower parts of the Slate Creek Complex correlate with plutons (ca. 200 Ma) and the volcanic cover sequence of the Rattlesnake Creek terrane in the Klamath Mountains. The uppermost volcanic unit correlates with the ca. 170 Ma Western Hayfork terrane that structurally overlies the Rattlesnake Creek terrane. Metamorphism in the Slate Creek Complex correlates broadly with a similar late metamorphic event in the Klamath Mountains.

FORMATION OF AMPHIBOLE AFTER CLINOPYROXENE BY DEHYDRATION REACTIONS: IMPLICATIONS FOR PSEUDOMORPHIC REPLACEMENT AND MASS FLUXES

The replacement of anhydrous by hydrous minerals is commonly used to infer infiltration of a rock by H 2 O. However, our work indicates that the formation of pseudomorphic amphibole after clinopyroxene (uralitization) in low-grade metavolcanic rocks is a net dehydration process. In the ophiolitic Slate Creek complex, northern California, clinopyroxene exhibits four textural stages of alteration: (1) clinopyroxene, (2) clinopyroxene + chlorite ± amphibole, (3) amphibole + chlorite, and (4) amphibole. This transformation occurs in subgreenschist to greenschist facies rocks with a common matrix assemblage: quartz, albite, chlorite, epidote, titanite, ± amphibole, ± white mica, ± rare pumpellyite. A simple reaction space model based on these observations indicates that consumption of chlorite in the rock matrix releases more water than required to produce amphibole pseudomorphs of clinopyroxene. Thus, the net flux of H 2 O during uralitization of greenschist facies metavolcanic rocks is outward. These results imply that uralitization in metavolcanic rocks results from heating rather than whole-rock hydration, and that natural mass fluxes may be counter to fluxes inferred from textural evidence alone.

Precambrian (?) crystallization and Permian (?) metamorphism of hypersolvus granite in the Avalonian terrane of Rhode Island: Summary

The igneous and metamorphic rocks of southeastern Massachusetts and Rhode Island occupy one of the most poorly understood geologic terranes in the northern Appalachians. Interest in this terrane has been renewed with the realization that it may contain importtant evidence concerning the opening and closing of the Atlantic Ocean and its predecessors (Wilson, 1966, 1973) and that some of the rocks in this region may be similar to the Avalon zone of Newfoundland (Williams, 1976; Robinson, 1976; Rast and others, 1976). In this paper, we describe the mineralogy and petrology of some Precambrian (?) granites in Rhode Island. Despite superimposed deformation and metamorphism, it is possible to identify relict igneous textures and to show that these granites formed from iron-enriched, alkaline magmas at a high level in the crust. High-grade meta-morphism and deformation has caused complete recrystallization of the feldspars in parts of the granite. Some of this metamorphism occurred during Permian time, but the effects of earlier episodes cannot be evaluated. Geologic Setting We have studied the Scituate Granite Gneiss (SGG), Hope Valley Alaskite Gneiss (HVA), and the Ten Rod Granite Gneiss (TRG) in southern and central Rhode Island (Quinn, 1971). The age of these granites is probably upper Precambrian or lower Paleozoic based on several whole-rock Rb-Sr isochrons. Day (1968 and unpub. data) has made preliminary analyses that suggest an age of about 600 m.y. for the three units. These preliminary data are supplemented by reports of granites with similar ages from nearby areas (Fairbairn and others, 1967; Hills and Dasch, 1972; Smith and Giletti, 1977; Kovach and others, 1977).