Ultramafic rocks at the Isua supracrustal belt and East Pilbara Terrane are crustal cumulates, not slices of early mantle

2020 ◽  
Author(s):  
Jiawei Zuo ◽  
Alex Webb ◽  
Jason Harvey ◽  
Peter Haproff ◽  
Thomas Mueller ◽  
...  
Keyword(s):  
Trace Element ◽  
Plate Tectonics ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Ultramafic Rocks ◽  
Depleted Mantle ◽  
Supracrustal Belt ◽  
Element Fractionation ◽  
Trace Element Fractionation ◽  
Geochemical Studies

<p>The initiation of plate tectonics remains enigmatic, with the proposed onset timing ranging from Hadean to Proterozoic. Recently, many mineralogical, petrological and geochemical studies suggest onset of plate tectonics at ~3 Ga. For example, the geology of East Pilbara Terrane (~3.55 to 2.70 Ga; Australia) is widely interpreted as representing Paleoarchean non-plate tectonics, followed by plate tectonics after a ~3.2 Ga transition. In contrast, Isua supracrustal belt (3.85 to 3.55 Ga; Greenland) has been dominantly interpreted via plate tectonics. There, two ultramafic lenses have been interpreted as depleted mantle slices, emplaced via thrusting in an Eoarchean subduction zone, implying early plate tectonics. We present new petrological and geochemical data of ultramafic samples from the Isua lenses and from the East Pilbara Terrane to explore their origins. Pilbara samples appear to preserve cumulate textures; protolith textures of Isua samples are altered beyond recognition. Samples with low chemical alteration show similar whole-rock chemistry, including up to 5.0 wt.% Al<sub>2</sub>O<sub>3</sub> and up to 0.25 wt.% TiO<sub>2</sub> that both covary negatively with MgO (37.1 to 47.5 wt.%); these variations suggest cogenetic relationships with local lavas. Flat trace-element fractionation trends parallel those of local lavas in the primitive-mantle normalized spider diagram. Spinel crystals from Pilbara samples yield ~20-60 Mg#, relatively constant Cr# at ~70, and 0.61-4.81 wt.% TiO<sub>2</sub>. Our data are consistent with crustal cumulate emplacement. In contrast with depleted mantle rocks, our samples have higher whole-rock Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub>, flat (vs. upward) trace-element fractionation trends from less to more compatible elements, and spinel crystals with higher TiO<sub>2</sub> and relatively constant (vs. varied) Cr#. Therefore, Isua and Pilbara ultramafic rocks may have similar, non-plate tectonic origins, and the Isua record allows a ~3 Ga onset of plate tectonics.</p>

Trace element fractionation processes in the generation of island arc basalts

1993 ◽  
Vol 342 (1663) ◽  
pp. 179-191 ◽  
Keyword(s):  
Trace Element ◽  
Local Equilibrium ◽  
Crustal Thickness ◽  
Distribution Coefficients ◽  
Island Arc ◽  
Source Rocks ◽  
Fluid Distribution ◽  
Geochemical Data ◽  
Element Fractionation ◽  
Trace Element Fractionation

Subduction-related magmas are characterized by distinctive minor and trace element ratios which are widely attributed to the introduction of a hydrous component from the subducted crust. Island arc rocks may usefully be subdivided into high and low Ce/Yb groups, and the latter are characterized by relatively restricted radiogenic isotope ratios. In general, high LIL/ HFSE ratios are best developed in low HFSE rocks, and the variation in LILE is less than that in HFSE. A local equilibrium model is developed in which the distinctive minor and trace element feature of arc rocks are the result of fluid percolation in the mantle wedge. Peridotite/fluid distribution coefficients are inferred to vary systematically with ionic radius in the range 69—167 x 10 -12 m. However, in practice the calculated olivine/fluid partition coefficients are too high to develop an arc signature in the wedge peridotite in reasonable timescales, and for acceptable fluxes from the slab. The available geochemical data would suggest that realistic distribution coefficients are 2-3 orders of magnitude less than those presently available from experimental data, presumably because the fluid compositions are different, or that local equilibrium is not appropriate. Average compositions from the low Ce/Yb arc suites exhibit a positive correlation between Ce/Sm, but not K/Sm, and crustal thickness. It is argued that the degree of melting varies with crustal thickness, but not in any simple way with the magnitude of the fluid contribution. The observed range in Ce/Sm in the low Ce/Yb rocks is consistent with 3-18% melting of slightly lree depleted source rocks.

scholarly journals Nd-Hf isotope geochemistry and lithogeochemistry of the Rambler Rhyolite, Ming VMS deposit, Baie Verte Peninsula, Newfoundland: evidence for slab melting and implications for VMS localization

2021 ◽  
Author(s):  
S J Piercey ◽  
J -L Pilote
Keyword(s):  
Rare Earth ◽  
Trace Element ◽  
Isotope Geochemistry ◽  
Primitive Mantle ◽  
Isotopic Data ◽  
Depleted Mantle ◽  
Element Enrichment ◽  
Felsic Volcanic ◽  
Magmatic History ◽  
Felsic Rocks

New high precision lithogeochemistry and Nd and Hf isotopic data were collected on felsic rocks of the Rambler Rhyolite formation from the Ming volcanogenic massive sulphide (VMS) deposit, Baie Verte Peninsula, Newfoundland. The Rambler Rhyolite formation consists of intermediate to felsic volcanic and volcaniclastic rocks with U-shaped primitive mantle normalized trace element patterns with negative Nb anomalies, light rare earth element-enrichment (high La/Sm), and distinctively positive Zr and Hf anomalies relative to surrounding middle rare earth elements (high Zr-Hf/Sm). The Rambler Rhyolite samples have epsilon-Ndt = -2.5 to -1.1 and epsilon-Hft = +3.6 to +6.6; depleted mantle model ages are TDM(Nd) = 1.3-1.5 Ga and TDM(Hf) = 0.9-1.1Ga. The decoupling of the Nd and Hf isotopic data is reflected in epsilon-Hft isotopic data that lies above the mantle array in epsilon-Ndt -epsilon-Hft space with positive ?epsilon-Hft values (+2.3 to +6.2). These Hf-Nd isotopic attributes, and high Zr-Hf/Sm and U-shaped trace element patterns, are consistent with these rocks having formed as slab melts, consistent with previous studies. The association of these slab melt rocks with Au-bearing VMS mineralization, and their FI-FII trace element signatures that are similar to rhyolites in Au-rich VMS deposits in other belts (e.g., Abitibi), suggests that assuming that FI-FII felsic rocks are less prospective is invalid and highlights the importance of having an integrated, full understanding of the tectono-magmatic history of a given belt before assigning whether or not it is prospective for VMS mineralization.

scholarly journals The Tommot pluton: a Middle Paleozoic rift-related alkaline gabbro and syenite complex, Yakutia, northeast Russia

2009 ◽  
Vol 4 ◽  
pp. 97-109
Author(s):  
V. A. Trunilina ◽  
P. W. Layer ◽  
L. M. Parfenov ◽  
A. I. Zaitsev ◽  
Y. S. Orlov
Keyword(s):  
Ultramafic Rocks ◽  
Discrete Events ◽  
Northeast Russia ◽  
Alkaline Rocks ◽  
Small Complex ◽  
Marginal Part ◽  
Depleted Mantle ◽  
Middle Paleozoic ◽  
Internal Deformation ◽  
Geochemical Studies

Abstract. The Tommot pluton is located within the continental Omulevka terrane of the inner zone of the Verkhoyansk-Kolyma Mesozoic orogen. It is a small complex (~12 km2) composed of alkaline-ultramafic rocks, alkaline and subalkaline gabbroids, and alkaline and quartz syenites. The pluton is unique both in the composition and age of its constituent rocks. Mineralogical-petrographical and geochemical studies of the rocks indicate that the alkaline rocks resulted from the melting of depleted mantle horizons. K-Ar, Rb-Sr, and 40Ar/39Ar age determinations confirm a Paleozoic age of the rocks. Formation of the alkaline rocks is related to Middle Paleozoic rifting which occurred as two discrete events: a Late Devonian event, which affected the marginal part of the Siberian continent, and a Late Carboniferous event that reflects internal deformation of the Omulevka terrane or late-stage extension. A spatially associated alkali granite, the Somnitel'nyy pluton, is Late Jurassic–Early Cretaceous in age and is synchronous with accretion of the Kolyma-Omolon Superterrane to Siberia in the Mesozoic.

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