Permian to cretaceous granites and felsic volcanics from SW Vietnam and S Cambodia: Implications for tectonic development of Indochina

Zircon ages and geochemistry are presented for igneous rocks from SW Vietnam and S Cambodia. Four main age groupings occur: Cretaceous (107–75 Ma), Early Jurassic (195 Ma), Late Triassic (230–222 Ma), and Permian (294–265 Ma). Cretaceous and Jurassic samples are amphibole-bearing biotite granodiorites to biotite granites and occur mainly east of the Kampot Fold Belt. Pre-Cretaceous samples occur within the Kampot Fold Belt and are dominantly felsic volcanics. The rocks are primarily high-K calc-alkaline, weakly peraluminous rhyolites or granites, with similar arc-like trace element patterns. Cretaceous granites are similar to Dalat Zone Granites and formed during Pacific subduction. Two Cretaceous granites have adakite-like signatures. The youngest Cretaceous granite has an A-type signature and may represent post-collision activity following the cessation of subduction. Jurassic granites are also linked to Pacific subduction. Two Triassic samples could be affiliated to similarly aged rocks in the Chanthaburi Terrane, or linked to the Loei Fold Belt and related to closure to the Sa Kaeo back arc. We cannot exclude that they are related to Pacific Plate subduction. Correlation of Permian volcanics to paleo-Tethys subduction and the Sukhothai-Chanthaburi arc is not clear as the Cambodian volcanics are older than Chanthaburi rocks, and there are stratigraphic contrasts between S Cambodia and Chanthaburi. Furthermore, such a correlation requires extension of the Sa Kaeo suture into S Cambodia and SW Vietnam at a high angle to regional structures. We propose that the Permian volcanics are also related to Paleo-Pacific subduction as shown in regional reconstructions of the region

Os-186 and Os-187 Enrichments and High-He-3/He-4 sources in the Earth's Mantle: Evidence from Icelandic Picrites

Picrites from the neovolcanic zones in Iceland display a range in Os-187/Os-188O from 0.1297 to 0.1381 ((gamma)Os = 0.0 to 6.5) and uniform Os-186/Os-188 of 0.1198375+/-32 (2 (sigma)). The value for Os-186/Os-188 is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398+/-16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in Os-186/Os-188 and Os-187/Os-188 from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high He-3/He-4, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not. A positive correlation between Os-187/Os-188 and He-3/He-4 from 9.6 to 19 RA in Iceland picrites is best modeled as mixtures of 500 Ma or older ancient recycled crust mixed with primitive mantle, creating a hybrid source region that subsequently mixes with the convecting MORB mantle during ascent and melting. This multistage mechanism to explain these isotope systematics is consistent with ancient recycled crust juxtaposed with more primitive, relatively He-rich mantle, in convective isolation from the upper mantle, most likely in the lowermost mantle. This is inconsistent with models that propose random mixing between heterogeneities in the convecting upper mantle as a mechanism to explain the observed isotopic variation in oceanic lavas or models that produce a high He-3/He-4 signature in melt depleted and strongly outgassed, He-poor mantle. Instead these systematics require a deep mantle source to explain the 3He/4He signature in Iceland lavas. The He-3/He-4 of lavas derived from the Iceland plume changed over time, from a maximum of 50 RA at 60 Ma, to approximately 25-27 RA at present. The changes are coupled with distinct compositional gaps between the different aged lavas when H-3/He-4 is plotted versus various geochemical parameters such as Nd-143/Nd-144 and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period

Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy

Recent basaltic andesite lavas from Merapi volcano contain abundant, complexly zoned, plagioclase phenocrysts, analysed here for their petrographic textures, major element composition and Sr isotope composition. Anorthite (An) content in individual crystals can vary by as much as 55 mol% (An40-95) across internal resorption surfaces with a negative correlation between high An mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analyses of zoned plagioclase phenocrysts show that the 87Sr/86Sr ratios of individual zones range from 0·70568 to 0·70627. The upper end of this range is notably more radiogenic than the host basaltic andesite whole-rocks (< 0·70574). Crystal zones with the highest An content have the highest 87Sr/86Sr values, requiring a source or melt with elevated radiogenic Sr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptive rocks contain abundant xenoliths, including metamorphosed volcanoclastic sediment and carbonate country rock (calc-silicate skarns) analysed here for petrographic textures, mineralogy, major element composition and Sr isotope composition. The xenoliths contain extremely calcic plagioclase (up to An100) and have whole-rock 87Sr/86Sr ratios of 0·70584 to 0·70786. The presence of these xenoliths and their mineralogy and geochemistry, coupled with the 87Sr/86Sr ratios observed in different zones of individual phenocrysts, indicate that magma-crust interaction at Merapi is potentially more significant than previously thought, as numerous crystal cores in the phenocrysts appear to be inherited from a metamorphosed sedimentary crustal source. This has potentially significant consequences for geochemical mass-balance calculations, volatile saturation and flux and eruptive behaviour at Merapi and similar island arc volcanic systems elsewher

Dating agpaitic rocks : a multi-system (U/Pb, Sm/Nd, Rb/Sr and 40Ar/39Ar) isotopic study of layered nepheline syenites from the Ilímaussaq complex, Greenland

This research was funded by the Carlsberg Fund [grant nr. 2013_01_0191 to TW] and Geocenter Denmark [GreenCrimi project, grant nr. 4-2012 to AMB]. Quadlab is funded by a grant from the Villum Foundation to MS. AMB and AF thank the NERC SoS RARE project [grant nr. This research was funded by the Carlsberg Fund [grant nr. 2013_01_0191 to TW] and Geocenter Denmark [GreenCrimi project, grant nr. 4-2012 to AMB]. Quadlab is funded by a grant from the Villum Foundation to MS. AMB and AF thank the NERC SoS RARE project [grant nr. NE/M010856/1] for support during the writing of this manuscript. ] for support during the writing of this manuscript.The Ilímaussaq complex in southern Greenland is a shallow crustal composite intrusion comprising augite syenite, peralkaline granite and volumetrically dominant agpaitic nepheline syenites. Previous studies indicated a baddeleyite U-Pb age of 1160 ± 5 Ma for the augite syenite, the earliest intrusive unit of the complex. A similar crystallization age, within error, is inferred for the main sequence of agpaitic nepheline syenites. However, direct age determination of these units has been challenging because agpaitic rocks characteristically lack robust phases for in situ U-Pb dating (e.g. zircon/baddeleyite). An additional challenge is the pervasive subsolidus alteration, of which the isotopic effects are poorly constrained. Here we present new U-Pb, Sm-Nd and Rb-Sr isotopic data from whole rocks and mineral separates and a 40Ar/39Ar amphibole age of three co-genetic agpaitic nepheline syenites (kakortokite) from the lowermost exposed part of the complex. Using a multi-system geochronological approach for mineral separates and whole rocks, we explore the effects of late-stage alteration for each isotopic system. Assuming a closed-system evolution for the hydrothermal fluids (i.e. isotopically similar to the melts) and cooling within a relatively short time-frame (<0.8 Ma), we evaluate whether traditional mineral-whole rock isochron methods can provide useful age constraints for agpaitic rocks. We compare our data with those in the literature, corrected for the most recent decay constants. Single-crystal40Ar/39Ar step-heating experiments yield an amphibole plateau age of 1156.6 ± 1.4 Ma (MSWD = 1.5, external error ± 7.7 Ma), which we put forward as the most precise crystallization age for the agpaitic units to date. Kakortokite whole rock and mineral separates (amphibole, eudialyte, feldspar) yield a 206Pb-207Pb isochron age of 1159 ± 17 Ma (MSWD = 0.96) and a 235U-207Pb isochron age of 1168.5 ± 8.8 Ma (MSWD = 0.82). These are within error of the baddeleyite and zircon U-Pb ages from the augite syenite and alkali granite, as well as the new plateau age, if we take into account the external error of 7.7 Ma (i.e. accuracy). The 235U-207Pb age thus far provides the best non-single mineral age estimate for the agpaitic suite. Sm-Nd isotopes for the same whole rock and mineral separates yield an isochron age of 1156 ± 53 Ma (MSWD = 0.23) with εNdi = −0.8 ± 0.8, with significantly less scatter than Nd data for the rest of the complex. Rb-Sr isotopes yield errorchron ages that are either unrealistically young (3-point feldspar errorchron: 1106 ± 9 Ma, suggesting partial 87Sr loss), or old (WR, amphibole and eudialyte: 1237 ± 21 Ma, n = 9). The data demonstrate that the U-Pb and Sm-Nd systems are relatively insensitive to late-magmatic alteration and re-equilibration during cooling. In contrast, the Rb-Sr system records significant disturbance, reflecting the highly mobile nature of Rb and Sr in peralkaline systems. This warrants careful reconsideration of previously published Rb-Sr isochron data, and caution in interpreting Rb-Sr data for other peralkaline complexes. Initial isotopic compositions for the kakortokite support petrogenetic models that describe Ilímaussaq melt evolution towards strongly radiogenic Sr values at relatively constant εNdi, with progressive evolution from the early augite syenite to the most fractionated agpaitic melts. The melts experienced variable but minor degrees of lower crustal assimilation and preferential leaching of radiogenic Sr from the Proterozoic granitic country-rock.PostprintPeer reviewe

Structural Controls on Shallow Cenozoic Fluid Flow in the Otago Schist, New Zealand

The Otago Schist in the South Island of New Zealand represents an exhumed Mesozoic accretionary prism. Two coastal areas (Akatore Creek and Bruce Rocks) south of Dunedin preserve structural and geochemical evidence for the development of postmetamorphic hydrothermal systems that involved widespread fluid-rock reaction at shallow crustal depths. The Jurassic to Triassic pumpellyite-actinolite (Akatore Creek) to upper greenschist facies (Bruce Rocks) metamorphic fabrics were crosscut by sets of regionally extensive Cretaceous exhumation joints. Many of the joints were subsequently reactivated to form networks of small-displacement (<metres) strike-slip faults containing cemented fault breccias and veins composed of hydrothermal calcite, siderite, and ankerite. Paleostress analysis performed on infrequent fault slickenlines indicates an overall strike-slip paleostress regime and a paleo-σ1 orientation (azimuth 094°) similar to the contemporary σ1 orientation in Otago and Canterbury (azimuth c. 110°-120°). High δ18O values in vein calcite (δ18OVPDB=21 to 28‰), together with the predominance of Type I calcite twins, suggest that vein formation occurred at low temperatures (<200°C) in the shallow crust and was associated with strongly channelized fluid flow along the joint and fault networks. Mass-balance calculations performed on samples from carbonate alteration zones show that significant mobilisation of elements occurred during fluid flow and fluid-rock reaction. Whole-rock and in situ carbonate 87Sr/86Sr data indicate varying degrees of interaction between the hydrothermal fluids and the host rock schists. Fluids were likely derived from the breakdown of metamorphic Ca-rich mineral phases with low 87Rb in the host schists (e.g., epidote or calcite), as well as more radiogenic components such as mica. Overall, the field and geochemical data suggest that shallow fluid flow in the field areas was channelized along foliation surfaces, exhumation joints, and networks of brittle faults, and that these structures controlled the distribution of fluid-rock reactions and hydrothermal veins. The brittle fault networks and associated hydrothermal systems are interpreted to have formed after the onset of Early Miocene compression in the South Island and may represent the manifestation of fracturing and fluid flow associated with reverse reactivation of regional-scale faults such as the nearby Akatore Fault