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Jurassic formation and Early-Oligocene high-pressure metamorphism of the Voltri Massif ophiolites: U-Pb age constraints from eclogite-facies rocks
No full textU-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif, Western Alps)
Full text linkU-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160 ± 1 and 161 ± 3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain baddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodo-luminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3-1.7 wt% , traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6 ± 1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary
The mafic rocks of Shao la (Kharta, S. Tibet): Ordovician basaltic magmatism in the Greater Himalayan Cristalline of Central-Eastern Himalaya
No full textIn the Kharta area, east of Mount Everest, the Greater Himalayan Crystallines are significantly richer in
mafic rocks than the surrounding areas, Sikkim–West Bhutan and Makalu–Cho Oyu. These rocks are
lenses with a complex metamorphic history. The mafic lenses of Shao La, in the Greater Himalayan
Sequence south of Kharta, are here considerated as dismembered dykes apparently escaped the Himalayan
high-temperature metamorphism and only record a low-grade metamorphic event. They are calcalkaline
medium-K basalts to basaltic andesites, consisting of plagioclase (core 62% An and rim 55%
An), augite (Wo43–47En3636–37Fs16–20), hypersthene (Wo1.6–3.3En50–52Fs46–48), and minor brown hornblende,
biotite and ilmenite. They show strong enrichment in low ionic potential elements relative to
high-field-strength elements, and only minor Ce and P enrichment with respect to MORB. Combined
Sr–Nd systematics suggest contamination of a basic magma from a subcontinental mantle source with
a small amount of crust (about 4 vol.%). This in turn indicates that the Shao La basalts and basaltic andesites
have the geochemical fingerprint of a supra-subduction zone magma.
U–Pb dating of zircon from one sample yielded an age of 457 ± 6 Ma for the crystallisation of the Shao
La basic rocks, assigning them to the Cambro-Ordovician Bhimphedian orogenic event. The age and geochemical
characteristics of the Shao La rocks are similar to those of the basic rocks of the Cambro-Ordovician
Mandi pluton further west. This suggests the existence of an extensive supra-subduction zone
magmatism along the Indian margin of Gondwana. Like the bimodal granite-gabbro magmatism in the
Mandi-Kaplas area, the Shao La basic rocks are contemporaneous with the emplacement of granitic plutons
in the Everest-Kharta area. This acid plutonism is interpreted as crustal melt triggered by the
upwelling of metasomatised mantle in a back-arc setting. The age of basic and acidic plutonism in the
Everest-Kharta area is evidence that the Bhimphedian Orogeny was still active as late as the Late
Ordovicia
Do extrusion ages reflect magma generation processes at depth? An example from the Neogene Volcanic Province of SE Spain
Full text linkThe high-K calc-alkaline volcanic rocks along the Neogene Volcanic Province of SE Spain represent crustal anatectic melts mixed with mantle components during the opening of the Alboran Sea. Partially melted metapelitic enclaves, along with the geochemical signature, provide evidence of their crustal source. U-Pb SHRIMP geochronology on monazite and zircon from enclaves and their hosting lavas in the localities of El Hoyazo, MazarrA(3)n and Mar Menor reveals variable delays between the melting at depth and the eruption of the volcanics. These data indicate that: (1) the most important event of anatexis in the Neogene spanned at least the 3 m.y. interval between 12 and 9 Ma; (2) there is no trend in age of crustal melting; and (3) the delay between magma generation and extrusion varies from more than 3 m.y. at El Hoyazo to similar to 0.5 m.y. and possibly 2.5 m.y. at Mar Menor, with no significant delay measurable at MazarrA(3)n. The variable time delay between anatexis and lava extrusion indicates that radiometric ages of volcanics may provide misleading information on the timing of magma genesis occurring at depth. This highlights the pitfall of basing detailed geodynamic models on volcanic extrusion ages alone
Extensional shear zones in the core of the Higher Himalayan Crystallines (Bhutan Himalayas): Evidence for extrusion?
No full textRecent fieldwork in western Bhutan, dedicated to unravelling the tectonic structure of the mid-crustal rocks, indicates a complex deformation pattern in the Greater Himalayan Slab (GHS). A system of normal shear zones, striking NE-SW and steeply to moderately dipping to the SE, has been recognized within this extruding slab or wedge of crystalline rocks. The zones are characterized by well developed shear-sense indicators pointing to a top-down-to-SE sense of shear. The main Barrovian metamorphic minerals are bent and stretched by extensional shear bands and associated deformation mechanisms indicate a range of brittle-ductile deformation conditions. Normal shear zones are concentrated in the middle-upper part of the GHS and indicate a thrust-transport-parallel lengthening of the core itself. Vorticity analysis highlights a non-coaxial flow with pure and simple shear acting together during deformation (mean vorticity number bracketed between 0.63 and 0.76). These data, when compared to available data near the tectonic boundaries of the GHS, indicate an increasing component of pure shear towards the interior of the GHS. The ages of zircon overgrowths and monazites from a slightly deformed granite, 20.5 ± 0.5 Ma, and a mylonitic granite deformed into the shear zones, 17.0 ± 0.2 Ma, bracket the age of shear zone formation at close to 17 Ma. According to our data, the normal shear zones could well accommodate the pure shear component of deformation localized in the inner part of the extruding wedge/slab and is compatible with a channel flow model
Extensional shear zones in the core of the Higher HimalayanCrystallines (Bhutan Himalayas): evidence for extrusion?
No full textDifferent generations of higher himalayan leucogranites in western Bhutan and their tectonic setting
No full textWas the exhumation of the Greater Himalayan Sequence in central Himalayas totally driven by STD and MCT?
No full textLeucogranite intruding the South Tibetan Detachment in western Nepal: implicarions for exhumation models in the Himalayas
No full textThe most popular models regarding the exhumation of the
Greater Himalayan Sequence (GHS), such as extrusion, channel
flow, critical taper and wedge extrusion, require prolonged
activity of the two bounding shear zones and faults, the Main
Central Thrust (MCT) and the South Tibetan Detachment
(STD). We present the crystallization age of an undeformed
leucogranite that intrudes both the GHS and the Tethyan
Himalaya Sequence (THS). Zircon and monazite U-Pb ages in
the leucogranite give ages between 23 and 25 Ma constraining,
at that time, the end of shearing along the STD. Our
results limit the contemporaneous activity of the MCT and
STD to a short period of time (~1–2 Ma) and thus argue
against exhumation models requiring prolonged contemporaneous
activity of the MCT and STD
Chemical and Isotopic Exchanges in Serpentinites during Hydration and Dehydration Events in the Erro-Tobbio Massif (Italy): from Ocean to Subduction
No full textSubducted serpentinites are important for recycling water and fluid mobile elements into the deep Earth. Here we present a texturally controlled in situ oxygen and boron isotope and trace element study of the Erro-Tobbio ultramafic rocks (Western Alps,Italy) that have experienced exhumation to the seafloor, hydration and subsequent dehydration upon subduction. High variability in δ18O from +0.2 to +12.5 in the low-temperature serpentine polymorph lizardite coupled to strong enrichment in fluid mobile elements are associated to variable serpentinisation conditions at the ocean floor during the opening of the Piemonte-Liguria Ocean. Incipient oceanic serpentinisation, i.e.mesh formation after mantle olivine, occurred at approximately 160–325 ◦C ± 20–60◦C. Subsequently, mantle pyroxene was transformed into serpentine bastite with decreasing temperatures downto 75◦C to 100◦C. The transition metals V,Sc,Co,Zn and Mn, and the Ni/Cr in lizardite are indicative of precursor olivine or orthopyroxene. Remarkably, some lizardite in partially serpentinised and undeformed mantle peridotites remained metastable during subsequent subduction. During early subduction, the serpentine polymorph antigorite replaced lizardite in highly serpentinised ultramafic rocks. Antigorite shows a narrower δ18O range from +5.3 to +7.5 , indicating isotopic homogenisation at theregional scale. A redistribution of the transition metals is observed at the sample scale, and the fluidmobile elements B, Cl and Li are partly lost. The isotopic homogenisation and the element redistribution are likely the results of (i) sample internal equilibration during antigorite crystallisation and (ii) mobility of serpentinite-derived fluids during the lizardite-to-antigorite transition. During the peak subduction stage at 1.8 to 2.5 GPa and 550◦C to 650◦C, metamorphic olivine with δ18O of ∼+4 to+5 is formed from the dehydration of brucite+antigorite. This olivine is enriched in fluid mobile elements such as Li and B compared to primitive mantle and is in oxygen isotopic equilibrium with the co-existing antigorite. Based on this equilibrium and the coupled trace element systematics, there is no evidence of large influx of external slab fluids from other lithologies. Recrystallised mantle olivine haslower B and Li contents compared to metamorphic olivine formed during brucite dehydration,and has similar δ18O values, XMg and Ni/Mn as mantle olivine, and is not in isotopic equilibrium with the antigorite. Such metamorphic olivine produced by recrystallisation, as well as the metastable lizardite during subduction, cannot be used as indicators of fluid production from serpentine dehydration
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