Geochemisty and tectonic setting of igneous rocks in the Glenrock Station area,N.S.W.

In the Upper Murray Valley, Victoria, Late Silurian, high‐Si igneous rocks, which are closely associated with alkalic, basaltic dykes, were emplaced at high crustal levels following the peak of the Benambran Orogeny, which deformed and metamorphosed the Wagga Zone in Late Ordovician‐Early Silurian times. These rocks, which are informally termed ‘the Upper Murray high‐Si magmatic suite’, include leucogranites, rhyolite dykes and flows, and ash‐flow tuffs characterised by the following features. They are transitional from mildly peraluminous to mildly metaluminous; they represent relatively anhydrous magmas, in which halides were important volatile constituents; they have high Si, total alkalies, Rb, Th, U, Nb, Sn and heavy rare earth elements; and they are relatively repleted in Mg, Ca, Sr, Eu, V, Cr and Ni. In these respects and in their post‐orogenic setting and close association with alkalic basalts, they resemble many post‐orogenic granitoids from elsewhere. Such granitoids appear to have formed as partial melts during crustal extension following major episodes of deformation and high‐Si magmatism. A residual granulitic crust, from which an earlier generation of granitoid magmas had been extracted, is argued to be the source rock‐type for these post‐orogenic magmas. Tectonic extension, affecting such a crust, was accompanied by deep fracturing and basaltic vol‐canism. Mantle‐derived, CO₂‐ and halide‐rich fluids moved into the residual crust, causing widespread metasomatism, and emplacement of basaltic magma caused temperatures to rise until melting took place and a second group of magmas was produced. This model explains most aspects of the trace and major element chemistry of post‐orogenic, high‐Si igneous rocks and, for the Upper Murray high‐Si suite it also provides an explanation for variations in trace elements and isotopic characteristics. Other processes, such as crystal fractionation, magma mixing, thermogravi‐tational diffusion, and separation and loss of a volatile phase, provide explanations for variations within individual units of the suite, but they do not explain overall variations or the highly fractionated nature of the suite.     

Intra-arc extension in Central America: Links between plate motions, tectonics, volcanism, and geochemistry

This study revisits the kinematics and tectonics of Central America subduction, synthesizing observations of marine bathymetry, high-resolution land topography, current plate motions, and the recent seismotectonic and magmatic history in this region. The inferred tectonic history implies that the Guatemala–El Salvador and Nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension; extension arising from the interplay between subduction roll-back of the Cocos Plate and the ~ 10–15mm/yr slower westward drift of the Caribbean plate relative to the North American Plate. The ages of belts of magmatic rocks paralleling both sides of the current Nicaraguan arc are consistent with long-term arc-normal extension in Nicaragua at the rate of ~ 5–10mm/yr, in agreement with rates predicted by plate kinematics. Significant arc-normal extension can ‘hide’ a very large intrusive arc-magma flux; we suggest that Nicaragua is, in fact, the most magmatically robust section of the Central American arc, and that the volume of intrusive volcanism here has been previously greatly underestimated. Yet, this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits. Observed geochemical differences between the Nicaraguan arc and its neighbors which suggest that Nicaragua has a higher rate of arc-magmatism are consistent with this interpretation. Smaller-amplitude, but similar systematic geochemical correlations between arc-chemistry and arc-extension in Guatemala show the same pattern as the even larger variations between the Nicaragua arc and its neighbors.     

Kaguyak dome field and its Holocene caldera,Alaska Peninsula

Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka (¹⁴C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO₂). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO₂), and phreatic ejection of lakefloor sediments onto the caldera rim. CO₂ and H₂S bubble up through the lake, weakly but widely. Geochemical analyses (n = 148), including pre-and post-caldera lavas (53–74% SiO₂), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO₂). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.     

Radiolaria‐dated Lower Permian clastic‐rock sequence in the Fukuji area of the Hida‐gaien terrane,central Japan,and its inter‐terrane correlation across Southwest Japan

The stratigraphy and radiolarian age of the Mizuyagadani Formation in the Fukuji area of the Hida‐gaien terrane, central Japan, represent those of Lower Permian clastic‐rock sequences of the Paleozoic non‐accretionary‐wedge terranes of Southwest Japan that formed in island arc–forearc/back‐arc basin settings. The Mizuyagadani Formation consists of calcareous clastic rocks, felsic tuff, tuffaceous sandstone, tuffaceous mudstone, sandstone, mudstone, conglomerate, and lenticular limestone. Two distinctive radiolarian faunas that are newly reported from the Lower Member correspond to the zonal faunas of the Pseudoalbaillella u‐forma morphotype I assemblage zone to the Pseudoalbaillella lomentaria range zone (Asselian to Sakmarian) and the Albaillella sinuata range zone (Kungurian). In spite of a previous interpretation that the Mizuyagadani Formation is of late Middle Permian age, it consists of Asselian to Kungurian tuffaceous clastic strata in its lower part and is conformably overlain by the Middle Permian Sorayama Formation. An inter‐terrane correlation of the Mizuyagadani Formation with Lower Permian tuffaceous clastic strata in the Kurosegawa terrane and the Nagato tectonic zone of Southwest Japan indicates the presence of an extensive Early Permian magmatic arc(s) that involved almost all of the Paleozoic non‐accretionary‐wedge terranes in Japan. These new biostratigraphic data provide the key to understanding the original relationships among highly disrupted Paleozoic terranes in Japan and northeast Asia.     

Kema terrane: A fragment of a back-arc basin of the early Cretaceous Moneron–Samarga island-arc system, East Sikhote–Alin range, Russian Far East

Abstract The Kema terrane is a suite of Barremian(?)–Aptian to Albian volcano-sedimentary rocks of Sikhote–Alin that are interpreted as deposits of the back-arc basin of the Moneron–Samarga island-arc system. Compositional features of the different-type deposits indicate a near-slope depositional environment influenced by volcanic processes. Studies of slump fold orientation testify to the accumulation of material from southeast to northwest by gravitational sliding. Compositional characteristics of terrigenous rocks suggest the major provenance for detrital material was an ensialic volcanic island arc. Petrochemical characteristics of basaltic rocks indicate that the formations studied were confined to the back part of the arc.     

Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea

Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO₂ contents ranging from ～72 to 73 wt%, K₂O/Na₂O ratios of 1.04–1.23 and low MgO/FeO^t values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al₂O₃/Na₂O + K₂O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)_N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.7104–0.7106 and identical ¹⁴³Nd/¹⁴⁴Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of ?12. The depleted mantle model ages (T_DM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.522–18.552, ²⁰⁷Pb/²⁰⁴Pb = 15.642–15.680 and ²⁰⁸Pb/²⁰⁴Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.     

Holocene volcanic activity at Koniuji Island,Aleutians

Reconnaissance mapping and ⁴⁰Ar/³⁹Ar age determinations establish an eruptive chronology for Koniuji Island in the central Aleutian island arc. Koniuji is a tiny 0.95 km² island that rises only 896 ft above the Bering Sea. Previous accounts describe Koniuji as a mostly submerged, deeply eroded, dormant stratovolcano. However, new ⁴⁰Ar/³⁹Ar ages constrain the duration of subaerial eruptive activity from 15.2 to 3.1 ka. Furnace incremental heating experiments on replicate groundmass separates from two samples of a 30–50 m thick basaltic andesite flow at the southernmost point of the island gave a weighted mean ⁴⁰Ar/³⁹Ar age of 15.2 ± 5.0 (2σ). The next phase of eruptive activity includes a series of 5.8–4.6 ka basaltic andesitic to andesitic lava flows preserved along the western shoreline. The basal lavas contain numerous mafic enclaves and dioritic cumulates suggesting a major disturbance in the plumbing system during the initial stages of emplacement. The 5.8–4.6 ka lavas are truncated by an andesitic dome complex that includes hornblende-bearing domes, flows and pyroclastics which extruded into the center of the island and comprise the majority of the subaerial eruptive volume. An angular block from within the dome complex yielded ⁴⁰Ar/³⁹Ar age of 3.1 ± 1.9 ka, thereby making it one of the youngest island arc volcanics to be dated using the ⁴⁰Ar/³⁹Ar method. Overall, the ⁴⁰Ar/³⁹Ar data indicate that Koniuji is a nascent stratovolcano that has only recently emerged above sea level, not a glacially-eroded, long-lived volcanic complex like those found on many other central Aleutian Islands.     

Adakitic lavas in the Central Luzon back‐arc region,Philippines: lower crust partial melting products?

Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.