David F. Siems

Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and “North America” blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78–1.69 Ga) and “North America” (1.71–1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace- element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the “North America” block are characterized by depleted eNd values (3.4–3.9) and younger Nd model ages (1800–1740 Ma) and have lower K 2 O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted eNd (0.6–2.6) and older Nd model ages (2070–1880 Ma). Despite the subtle differences, granitoids from both the Caborca and “North America” blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave– Yavapai transition zone, whereas rocks of the “North America” block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave–Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite, and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, among these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on the basis of distinctive rare-earth-element (REE) patterns: melilitite-related, melanephelinite-related, meymechite-related, trachybasalt-related, and ankaramite-related. The REE ratios and patterns that distinguish the groups have not evolved by fractionation, because they display no relation to MgO content. Judging from the isotopic data, crustal contamination had little influence on magma evolution. All rock groups, despite their geochemical dissimilarities, show close geochemical linkages among themselves, and significant geochemical similarity to kimberlites of the Yakutian province and ocean-island basalts (OIB). Thus, all of these continental and oceanic magmas may have originated in the same part of the mantle. Geochemical distinctions among the five rock groups could have been caused by various degrees of partial melting and differing amounts of dissolved volatiles.

Geochemical, Isotopic, and SHRIMP Age Data for Precambrian Basement Rocks, Permian Volcanic Rocks, and Sedimentary Host Rocks to the Ore-bearing Intrusions, Noril'sk-Talnakh District, Siberian Russia

Petrographic, geochemical, and isotopic data have been obtained for 33 samples selected to provide constraints on contamination models for the volcanic and intrusive components of the Late Permian to Early Triassic, Siberian flood-volcanic province. Twenty-one of these samples were carried from great depth in an explosive diatreme of Triassic age, whereas 12 were collected from drill core from depths of tens to 2000 m. The studied diatreme xenoliths are: (1) fragments of the crystalline basement; and (2) fragments of a basaltic-to-rhyolitic volcanic suite. Prompted by an unexpected, Late Paleozoic, Rb-Sr isochron age for this compositionally diverse volcanic suite, a SHRIMP U-Pb zircon age of ∼270 Ma was obtained for a rhyodacite xenolith. Previously, a SHRIMP zircon U-Pb age of ∼910 Ma had been determined for a leucogranite xenolith from the crystalline basement; this sample also contains substantial amounts of inherited, Early Proterozoic and Archean zircon. The presence of this volcanic suite, only ∼20 m.y. older than the 251 Ma, flood-volcanic sequence, is an extremely provocative result, inasmuch as hundreds of exploration drill holes in the Norilsk area, and throughout the Siberian platform, have encountered only Tungusskaya Series coal-bearing sedimentary rocks in this stratigraphic/time interval. These data support arguments that subduction/underthrusting from the West Siberian Lowland under the northwest margin of the Siberian craton took place in Late Permian time. The isotopic data obtained for the xenolith suite indicate that the upper part of the crystalline basement under the northwest margin of the Siberian craton is composed of Late Proterozoic (Riphean) rocks-alkaline granites, trondhjemites, crystalline schists, gneisses, and amphibolites-with much in common with rocks of the Central zone of the Taymyr folded area, which has been interpreted as an accretionary block formed and joined to Siberia in Late Riphean to Vendian time. Measured isotopic characteristics for the Precambrian crystalline basement, and the Paleozoic sedimentary rocks that host the ore-bearing intrusions in the Norilsk region, provide parameters for quantitative modeling of crustal contamination during evolution of the Siberian flood-volcanic rocks and related intrusions, both while en route to the surface and at the site of intrusion emplacement.

Volcanic Rocks from Rocas Alijos

The region offshore from southern and Baja California has an abundance of volcanic edifices. Most of the edifices are submarine seamounts or guyots but a few project above sea level. Most are small, conical volcanoes that are round or elliptical in plan view. Others are larger and more complex in shape, and some form elongated ridges (Fig. 1). Unlike many of the distinctive linear island chains of the central and southern Pacific, volcanic edifices offshore from the Californias may be isolated, clustered, or aligned in short chains. Short chains may show a NW trend compatible with direction of plate motion or they may show a variable NE to E trend that is at a high angle to the direction of plate motion.

Mount Mageik: A compound stratovolcano in Katmai National Park: A section in Geologic studies in Alaska by the U.S. Geological Survey, 1998

Mount Mageik is an ice-clad 2,165-m andesite-dacite stratovolcano in the Katmai volcanic cluster at the head of the Valley of Ten Thousand Smokes. New K-Ar ages indicate that the volcano is as old as 93+8 ka. It has a present-day volume of 20 km3 but an eruptive volume of about 30 km3, implying a longterm average volumetric eruption rate of about 0.33 km3 per 1,000 years. Mount Mageik consists of four overlapping edifices, each with its own central summit vent, lava-flow apron, and independent eruptive history. Three of them have small fragmental summit cones with ice-filled craters, but the fourth and highest is topped by a dacite dome. Lava flows predominate on each edifice; many flows have levees and ice-contact features, and many thicken downslope into piedmont lava lobes 50200 m thick. Active lifetimes of two (or three) of the component edifices may have been brief, like that of their morphological and compositional analog just across Katmai Pass, the Southwest (New) Trident edifice of 1953-74. The North Summit edifice of Mageik may have been constructed very late in the Pleistocene and the East Summit edifice (along with nearby Mount Martin) largely or entirely in the Holocene. Substantial Holocene debris avalanches have broken loose from three sites on the south side of Mount Mageik, the youngest during the Novarupta fallout of 6 June 1912. The oldest one was especially mobile, being rich in hydrothermal clay, and is preserved for 16 km downvalley, probably having run out to the sea. Mageiks fumarolically active crater, which now contains a hot acid lake, was never a magmatic vent but was reamed by phreatic explosions through the edge of the dacite summit dome. There is no credible evidence of historical eruptions of Mount Mageik, but the historically persistent fumarolic plumes of Mageik and Martin have animated many spurious eruption reports. Lavas and ejecta of all four component edifices of Mageik are plagioclaserich, pyroxene-dacites and andesites (57-68 weight percent SO2) that form a calcic, medium-K, typically low-Ti arc suite. The Southwest Summit edifice is larger, longer lived, and compositionally more complex than its companions. Compared to other centers in the Katmai cluster, products of Mount Mageik are readily distinguishable chemically from those of Mount Griggs, Falling Mountain, Mount Cerberus, and all prehistoric components of the Trident group, but some are similar to the products of Mount Martin, Southwest Trident, and Novarupta. The crater lake, vigorous superheated fumaroles, persistent seismicity, steep ice blanket, and numerous Holocene dacites warrant monitoring Mount Mageik as a potential source of explosive eruptions and derivative debris flows.