Charlotte M. Allen

TEMORA 1: a new zircon standard for phanerozoic U-Pb geochronology

The role of the standard is critical to the derivation of reliable U–Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation–inductively coupled plasma–mass-spectrometry (ELA–ICP–MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U–Pb intercomparisons between different laboratories that do not use a common spike.

Are A-type granites the high-temperature felsic granites? Evidence from fractionated granites of the Wangrah Suite

A‐type granites are a minor, but distinctive, component of the granites of the Lachlan Fold Belt of southeastern Australia. They are felsic rocks with SiO2 contents ranging from 69.7 to 77.1%, with an average of 73.8% (55 analyses). When unfractionated, as evidenced by high Ba contents, they are distinguished from felsic I‐type granites by a greater abundance of high‐field‐strength elements, such as Zr. The Wangrah Suite contains a diverse association of A‐type granites, comprising four main units with coherent geochemical trends overall, but with textural variation from equigranular through to porphyritic. The least felsic granites from the suite (Danswell Creek Granite ∼70% SiO2) have compositional features that suggest that they represent parental magma compositions. The most felsic granites (Dunskeig Granite ∼76% SiO2) were derived from such compositions by fractional crystallisation. The Wangrah Suite granites were emplaced at shallow levels (∼200 MPa), at high zircon saturation temperatures (>830°C) and relatively low water activity. The chemical composition of the Wangrah granites cannot be easily related to the adjacent mafic magmas. The compositionally variable Wangrah Suite differs from the homogeneous A‐type suites, such as the Gabo Suite to the southeast. Its variability is probably related to the efficiency of fractional crystallisation and emplacement along a major fault at shallow levels. We favour a single‐stage petrogenetic scheme where the A‐type magmas were produced by high‐temperature, partial melting of quartzo‐feldspathic crustal rocks. The relatively refractory nature of the source rocks may have been due to limited H2O content, relatively low fO2 and relatively high (TiO2 + FeOtotal)/MgO.

Brahmaputra sediment flux dominated by highly localized rapid erosion from the easternmost Himalaya

The Brahmaputra River slices an exceptionally deep canyon through the eastern Himalaya. Fission-track and laser-ablation U-Pb ages of detrital zircon grains from the river document very rapid erosion from this region and its impact on sediment fluxes downstream in the Brahmaputra. Downstream from the canyon, 47% of the detrital zircons in the rivers modern sediment load comprise a fission-track age population averaging only 0.6 Ma. Equally young cooling ages are reported from bedrock in the canyon through the Namche Barwa–Gyala Peri massif but are absent from riverbank sands of major tributaries upstream. Simple mixing models of U-Pb ages on detrital zircons from samples taken above and below this massif independently suggest that 45% of the downstream detrital zircons are derived from the basement gneisses extensively exposed in the massif. Constraints on the extent of the source area provided by bedrock cooling ages together with sediment-flux estimates at Pasighat, India, suggest exhumation rates averaging 7–21 mm yr −1 in an area of ~3300 km 2 centered on the massif. This rapid exhumation, which is consistent with the very young cooling ages of the detrital zircons from this area, produces so much sediment that ~50% of the vast accumulation in the Brahmaputra system at the front of the Himalaya comes from only ~2% of its drainage. This extreme localization of rapid erosion, sediment evacuation, and bedrock cooling bear on (1) common assumptions in geodynamic and geochemical studies of the Himalaya about sources of sediment, and (2) plans for hydroelectric development and flood management in southeastern Tibet and the heavily populated areas of eastern India.

Granite genesis and basin formation in an extensional setting: The magmatic history of the northernmost new england orogen

U‐Pb and K‐Ar age determinations on igneous rocks have been used to trace the Late Palaeozoic to Mesozoic magmatic and tectonic history of the northern New England Orogen, northeastern Australia. The oldest igneous rocks in the Connors Arch, tonalites of the Urannah Suite, yield zircon ages of 304.3 ± 5.8 and 308.2 ± 7.1 Ma. One of a swarm of felsic dykes chemically related to the Urannah Suite granites is distinctly younger at 283.9 ± 5.2 Ma. The cross‐cutting Thunderbolt Granite is slightly younger again at 277.9 ± 6.3 Ma. The granites of the Urannah Suite are not the remnants of a Late Devonian ‐ mid‐Carboniferous arc. A zircon age of 294.2 ± 2.8 Ma measured on an ignimbrite from the Carmila beds at Dumbleton Rocks indicates initiation of the adjacent Bowen Basin during, or soon after, the main Late Carboniferous batholith‐forming event. A later generation of more isotopically primitive magmatism is represented by younger plutons and mafic dykes. The Triassic Gloucester Granite east of the Connors Arch...

Thermochronology of mineral grains in the Red and Mekong Rivers, Vietnam: Provenance and exhumation implications for Southeast Asia

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U-Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210–240 Ma) than those in the Red River (230–290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700–800 Ma grains originally derived from the Yangtze Craton. 40Ar/39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150–200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350–500 Ma) eroded from the Yangtze Craton. High-grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet-Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5–3 km of the crust since ∼25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104–191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar-Ar and U-Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence.

U–Pb zircon geochronology of Late Devonian to Early Carboniferous extension‐related silicic volcanism in the northern New England Fold Belt*

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ∼15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.

Intrusive metallogenic provinces in eastern Australia based on granite source and composition

Ore element ratios in intrusion-related mineralisation are in part a function of the relative oxidation state and degree of fractionation of the associated granite suite. A continuum from Cu-Au through W to Mo dominated mineralisation related to progressively more fractionated, oxidised I-type magmas can be traced within single suites and supersuites. Such systematic relationships provide strong evidence for the magmatic source of ore elements in granite-related mineral deposits and for the production of the observed ore element ratios dominantly through magmatic processes. The distribution of mineralised intrusive suites can be used to define a series of igneous metallogenic provinces in eastern Australia. In general, there is a correlated evolution in the observed metallogeny (as modelled based on the compatibility of ore elements during fractionation) with increasing degree of chemical evolution of the associated magmatic suite. This is from Cu-Au associated with chemically relatively unevolved magmas, through to Sn and Mo-rich mineralisation associated with highly evolved magmas that had undergone fractional crystallisation. Provinces recognised in that way do not necessarily correlate with the tectonostratigraphic boundaries defined by the near-surface geology, indicating that the areal distribution of some granite source regions in the deep crust is unrelated to upper crustal geology.

Age of the Los Ranchos Formation, Dominican Republic: Timing and tectonic setting of primitive island arc volcanism in the Caribbean region

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island arc sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formation (Quita Sueno Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Sueno sample, formed at either 113.9 ± 0.8 or 118.6 ± 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 ± 0.8 Ma. The Cotui stock (RD-73-601) was emplaced ca. 111.8 ± 0.6 Ma or 112.9 ± 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calcalkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions.

Monsoon control over erosion patterns in the Western Himalaya: possible feed-back into the tectonic evolution

Abstract The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative ϵNd and higher 87Sr/86Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ13C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. Supplementary material: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412

A nested diapir model for the reversely zoned Turtle Pluton, southeastern California

Most zoned plutons described in the geological literature have mafic rims and felsic cores and are referred to as «normally zoned», whereas relatively few «reversely zoned» intrusions (felsic rims and mafic cores) have been described. That unusual zonation pattern has been variously attributed to in situ processes or to the reordering of an underlying, vertically stratified, magma chamber either by intrusion through an orifice or by emplacement of composite diapirs. The Turtle Pluton is an early Cretaceous, reversely zoned, intrusion that is divided into four facies: a Rim Sequence that is graditionally zoned from bt+ilm+musc monzogranite to hb+bt+mt+sph granodiorite