Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of the Late Silurian Shidonggou granite from the Wulonggou area in the Eastern Kunlun Orogen,Northwest China

ABSTRACT

The Wulonggou area in the Eastern Kunlun Orogen (EKO) in Northwest China is characterized by extensive granitic magmatism, ductile faulting, and orogenic gold mineralizations. The Shidonggou granite is located in the central part of the Wulonggou area. This study investigated the major as well as trace-element compositions, zircon U–Pb dates, and zircon Hf isotopic compositions of the Shidonggou granite. Three Shidonggou granite samples yielded an average U–Pb zircon age of 416 Ma (Late Silurian). The Late Silurian Shidonggou granite is peraluminous, with high alkali contents, high Ga/Al ratios, high (K₂O + Na₂O)/CaO ratios, and high Fe₂O₃^T/MgO ratios, suggesting an A-type granite. The Shidonggou granite samples have zircon ε_Hf(t) values ranging from ?7.1 to +4.4. The Hf isotopic data suggest that the Late Silurian granite was derived from the partial melting of Palaeo- to Mesoproterozoic juvenile mantle-derived mafic lower crust. Detailed geochronological and geochemical data suggest that the Late Silurian granite was emplaced in a post-collisional environment following the closure of the Proto-Tethys Ocean. Combining data of other A-type granitic rocks with ages of Late Early Silurian to Middle Devonian, such post-collisional setting related to the Proto-Tethys Ocean commenced at least as early as ~430 Ma (Late Early Silurian), and sustained up to ~389 Ma (Middle Devonian) in the EKO.     

U–Pb detrital zircon evidence of transcontinental sediment dispersal: provenance of Late Mississippian Wedington Sandstone member,NW Arkansas

ABSTRACT

U–Pb ages of detrital zircons from the Wedington Sandstone member in northwest Arkansas provide evidence for Late Mississippian westward transcontinental sediment transport from the Appalachian foreland. The Late Mississippian Wedington Sandstone member of the Fayetteville Shale is a fine- to medium-grained quartzarenite. It separates the Fayetteville Shale into informal lower and upper intervals, and was deposited as a small constructive delta complex that prograded towards the south and southeast during the Late Mississippian. As a major influx of clastic sediments, the Wedington Sandstone member records the sediment source and dispersal in the mid-continent during the Late Mississippian. A total of 559 detrital zircon grains from six Wedington samples were recovered for U–Pb detrital zircon geochronological analysis. Results show that age distributions can be subdivided into six groups: ~350–500, ~900–1350, ~1360–1500, ~1600–1800, ~1800–2300, and > ~2500 Ma, and are characterized by a prominent peak for the age group of ~900–1350 Ma, a major peak at ~1600–1800 Ma, and a few other minor age clusters. Regional correlation and geological evidence from surrounding areas suggest that the transcontinental sediment dispersal started as early as the Late Mississippian. U–Pb detrital zircon age distribution suggests that the Wedington Sandstone member was likely derived from the Appalachian foreland with contributions from the Nemaha Ridge to the west where the Yavapai–Mazatzal sources were exposed during the Late Mississippian. Sediment was likely transported westward through or around the Illinois Basin, merged with mid-continent sediment, and then entered into its current location in northwest Arkansas. Transportation of this sediment from mixed sources continued along its course to the south, forming a delta on the Northern Arkansas Structural Platform.     

Geochemistry and C–O and Nd–Sr isotope characteristics of the 2.4 Ga Hogenakkal carbonatites from the South Indian Granulite Terrane: evidence for an end-Archaean depleted component and mantle heterogeneity

ABSTRACT

The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [δ¹³C_VPDB = ?6.7 to ?5.8‰ and δ¹⁸O_VSMOW = 7.5–8.7‰ except a single ¹⁸O-enriched sample (δ¹⁸O = 20.0‰)] represent unmodified mantle compositions. The εNd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive εNd values, close to CHUR (εNd = ?0.35 to 2.94) and named high-εNd group while the low-εNd group samples show negative values (?5.69 to ?8.86), corresponding to depleted and enriched source components, respectively. The ⁸⁷Sr/⁸⁶Sr_i ratios of the two groups also can be distinguished: the high-εNd ones have low ⁸⁷Sr/⁸⁶Sr_i ratios (0.70161–0.70244) while the low-εNd group shows higher ratios (0.70247–0.70319). We consider the Nd–Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.     

A slab break-off model for mafic–ultramafic igneous complexes in the East Kunlun Orogenic Belt,northern Tibet: insights from early Palaeozoic accretion related to post-collisional magmatism

ABSTRACT

The East Kunlun Orogenic Belt (EKOB) in northern Tibet provides an important record of the amalgamation of the Wanbaogou oceanic basalt plateau and the Qaidam Block. Here we report geochemical, geochronological, and Hf isotopic data for newly identified late Silurian–Early Devonian mafic–ultramafic igneous complexes from the EKOB at the northern margin of the Tibetan Plateau. These complexes are dominantly composed of gabbro and pyroxenite rocks. Three complexes yield zircon U–Pb ages of 398.8 ± 1.8, 420.2 ± 1.2, and 413.4 ± 0.78 Ma. The ε_Hf(t) values of zircons range from +0.8 to +3.3 with T_DM1 ages of 897 to 998 Ma. Modelling of the geochemical data indicates that these igneous complexes have a hybrid origin, involving depleted mantle fluids derived from a previous subduction event and crustal materials. The geochemical and geochronological data suggest that these complexes formed in a post-collisional setting linked to break-off of a subducted oceanic slab, which occurred after the Wanbaogou oceanic basalt plateau amalgamated with the Qaidam Block in the late Silurian–Early Devonian.     

Early Cambrian alkaline volcanism on the southern margin of Laurentia: evidence in the volcaniclastic units from the Puerto Blanco Formation in the Caborca block,NW Mexico

ABSTRACT

Volcaniclastic units are exposed at the base of the Puerto Blanco Formation in the Caborca region, northwestern Mexico. The lower unit reveals the presence of Early Cambrian mafic volcanism in this region. It consists of a volcano-sedimentary sequence represented by tuffaceous conglomerates, agglomerates, lapillistones, tuffs, and altered mafic volcanic flows. Petrographic analysis classified the volcanic clasts as albite-sphene-calcite-actinolite granofels, with a moderate to intense hydrothermal alteration, precisely characterized by EPMA analysis. Albite-actinolite geothermometry indicates temperatures from 400 to 500°C, suggesting metamorphic conditions in the upper temperature greenschist facies. Geochemistry analysis shows a high TiO₂ basic–ultrabasic volcanism that originated the volcanic clasts. Rock protoliths were studied using immobile trace elements, which classified them as OIB-type alkaline basalts with the characteristic spider hump-shaped pattern, situated in an anorogenic intracontinental tectonic setting with enriched mantle signatures. ⁴⁰Ar/³⁹Ar geochronology shows metamorphic ages of 52.58 ± 2.0 and 91.67 ± 0.55 Ma, consistent with the emplacement of Laramidic granitoids identified in the region. Possible correlations of this alkaline volcanism include the Southern Oklahoma Aulacogen and the late stages of the rifting of north western Laurentia represented in western United States.     

Early Cretaceous granitic rocks from the southern Jiaodong Peninsula,eastern China: implications for lithospheric extension

ABSTRACT

Zircon U–Pb ages, major element and trace element compositions, and Sr, Nd, and Pb isotopic compositions for late Mesozoic granites from the southern Jiaodong Peninsula (eastern China) were determined. Ages for the Wulianshan, Xiaozhushan, and Dazhushan plutons are 119.1–122.3, 114.2, and 108.9 Ma, respectively. Major and trace element characteristics show that these granitic rocks belong to alkaline, A-type granites formed in an extensional setting. Trace element compositions show strong, variable negative anomalies in Ba, K, P and Ti, and positive anomalies in Rb, Th, U, Pb, Ce, Zr, and Hf, which are typical characteristics of A-type granites. Variable Sr and Nd isotopic compositions, ⁸⁷Sr/⁸⁶Sr(i) = 0.70540–0.7071 and εNd(t) = ?14.5 to ?20.9. Whole-rock Pb isotopic compositions have the following ranges, (²⁰⁶Pb/²⁰⁴Pb)t = 15.707–16.561, (²⁰⁷Pb/²⁰⁴Pb)t = 15.376–16.462, and (²⁰⁸Pb/²⁰⁴Pb)t = 36.324 to 37.064. Isotopic modelling indicates an origin that lies between mantle tapped by Cenozoic basalts around the Tan-Lu megafault and lower continental crust (LCC), and which can be explained by mixing of 11–18% mantle and 82–89% LCC. Based on new and compiled data, we suggest that the southern Jiaodong Peninsula, as well as the Laoshan area, was in a regional extensional setting of an orogenic belt during 106–126 Ma. The granitic rocks may be the result of late Mesozoic lithospheric thinning and decratonization (i.e. late Mesozoic craton destruction event occurring throughout eastern China).     

An analytical model of soil–structure interaction with swelling soils during droughts

Lightly loaded structures constructed on expansive soils may develop structural damage as a result of changes in the soil’s moisture content. This study investigated an analytical model of soil–structure interaction to assess the settlement of dwellings built on swelling soils when droughts occur. The building behavior was investigated with the Euler–Bernoulli beam theory, and the ground behavior was investigated with a Winkler-derived model based on the state surface approach. The analytical model results were compared to those of a finite element analysis using the Barcelona Expansive Model (BExM) performed with Code_Bright.The analytical model was then used to assess the settlement transmission ratio for a typology of clayey soils and different parameters of building. The results indicated that the final deflection of the building increased with the building length and soil suction. The building deflection due to the suction variations was inversely proportional to the load, the rigidity of the building and the embedding depth of the foundation. Increasing these parameters made the building less vulnerable to shrinkage and swelling action.     

A cave in Dolerite at Wayatinah,Tasmania

A cave (60 ft. x 30 ft. x 17 ft.) occurs within Jurassic dolerite 600 ft. from the surface. It was formed by zeolitization of a zone in the dolerite followed by solution of the secondary minerals to leave a cavity.     

Geochemical discrimination of tectonic setting for Devonian basalts of the Yarrol Province of the New England Orogen,central coastal Queensland: An empirical approach *

Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.     

Geochemical fingerprint of hyaloclasts in glassy fragmental rocks of Macquarie Island (Southern Ocean): implications for volcanogenic sedimentary processes at a waning mid-ocean ridge

Two main types of glassy fragmental rocks formed along the Proto-Macquarie Spreading Ridge: (i) hyaloclastite breccia; and (ii) pillow-fragment breccia. Examples now exposed on Macquarie Island, Southern Ocean, were largely sourced from proximal pillow lavas. In each of seven samples examined, hyaloclasts (basaltic glass grains) have a narrow major- and trace-element geochemical range, consistent with derivation of each sample from a single volcanic eruption event. Moreover, every sample analysed within the one stratigraphic section (at three sites) displays distinctive major- and trace-element geochemistry compared with the other two sections. This suggests that hyaloclasts at each site represent discrete magma batches. A single source for these glassy fragmental rocks contrasts with the dominant fault-scarp-derived polymict sedimentary rocks on Macquarie Island. We suggest that the hyaloclasts analysed in this study were deposited in small basins between the slopes of growing pillow cones along the mid-ocean ridge. The geochemical analyses presented here encompass (weakly) fractionated (e.g. Bauer Bay) to near-primitive (e.g. Pyramid Peak) compositions. All samples presented here lie within the range of the enriched- to normal-MORB suites previously reported for the island that include the least fractionated MORB melts known globally. The interpretation of geochemically distinct magma batches over the small area of the island suggests very limited magma mixing consistent with an immature or waning magmatic system. We relate these geochemical characteristics to: (i) volcanism near a very long offset transform; and (ii) genesis of magmas during the waning stages of slow seafloor spreading within a very narrow (<50 km-wide) spreading corridor.