Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

AbstractThis paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling

Geochemistry and Evolution of the Earth’s Mantle at the Hess Deep and the Mado Megamullion

Mantle peridotites have been widely sampled in the ocean basins. These are considered residues of the melting process that ultimately results in the formation of the oceanic crust. However, the exact mechanism for their evolution has not been comprehensively understood. Plagioclase-bearing mantle rocks, for example, may be formed by fractional crystallization, melt-rock reaction and assimilation in the crust and shallow mantle. Nearly all mid-ocean ridge basalts (MORB) are believed to have at least some melt-rock reaction process in their evolutionary history. Yet, there is a lack of direct evidence that captures the reaction textures or intermediate compositions of the melt-rock reaction process. Spreading centers and oceanic core complexes (OCC) provide a window to understand melt evolution processes in the lower crust and upper mantle. In this study we address the ongoing debate about the mechanism of melt transport in the mantle by textural and chemical analysis (major and trace elements) of samples from two unique tectonic regimes; a. the Hess Deep (fast spreading center in the Equatorial Pacific Ocean) and the Mado Megamullion (an OCC in the Shikoku back-arc basin). Magnetic anomaly data suggested that spreading in the Shikoku basin ceased after 15 Ma although the age of termination of spreading has not been constrained from geochronologic evidences until now. We integrated the model results with the natural samples to quantify the evolution of the peridotites. We dated zircons from amphibole-chlorite bearing veins that crosscut the peridotites of the Mado Megamullion. The U-Pb age of 13.37 ± 0.24 Ma and the trace and rare earth element (REE) chemistry of the zircons and the amphiboles suggests the possibility that magmatic activity and back-arc spreading at the Shikoku basin continued till 13 Ma. We could model the elevated TiO2 content in melt-reacted plagioclase-bearing peridotites from Hess Deep through assimilation fractional crystallization (AFC) process which could not be modelled by fractional crystallization process alone and has remained a topic of debate. Our results also show that back-arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those mid-ocean ridges or fore-arcs

Textures and compositions of cobalt pentlandite and cobaltian mackinawite from the Madan-Kudan copper deposit, Khetri Copper Belt, Rajasthan, India

The Khetri Copper Belt (KCB), a part of the Proterozoic Delhi–Aravalli fold belt in western India, hosts several Cu deposits, which are known to contain considerable Au, Ag, Co and Ni. Although many Co-bearing phases have been reported from the KCB and adjacent areas, detailed textural and geochemical data are either unavailable or scant except for mackinawite. In this study, we describe the textures and compositions (determined by EPMA) of two very rare Co-rich phases, namely cobaltian mackinawite (containing up to 12.68 wt.% Co, 1.90 wt.% Ni and 2.52 wt.% Cu) and cobalt-pentlandite (containing up to 49.30 wt.% Co and 10.19 wt.% Ni), identified based on composition, from the Madan-Kudan deposit. To the best of our knowledge, neither cobalt-pentlandite nor such highly Co-rich mackinawite have previously been reported from this area. The common sulphide minerals viz. chalcopyrite, pyrrhotite and rare pyrite occur in chalcopyrite-pyrrhotite ± pyrite-magnetite-chlorite-blue amphibole (Cl-rich hastingsite-pargasite-sadanagaite) ± marialitic scapolite ± allanite ± uraninite veins in amphibole-bearing feldspathic quartzite and garnetiferous chlorite schist. Cobaltian mackinawite is invariably associated with chalcopyrite and occurs as exsolution and inclusion within chalcopyrite or outside, but at the contact of chalcopyrite. On the other hand, cobalt-pentlandite is invariably associated with pyrrhotite and shows similar textural relation with pyrrhotite as that of mackinawite with chalcopyrite. Mineralogically diverse undeformed sulphide veins comprising Cl-rich amphibole and locally Cl-rich marialitic scapolite suggests epigenetic hydrothermal mineralization involving Cl-rich saline fluid in the Madan-Kudan deposit. Transport of metals, derived from a mafic source rock with high intrinsic Ni:Co ratio, by Cl-rich fluid can suitably explain the high Co:Ni ratio of the studied ore minerals. Presence of such highly Co-rich phases and other circumstantial evidences, enumerated in this work, are consistent with variants of Fe oxide (–Cu–Au) (IOCG) style mineralization, at least for some stages of mineralization in the Madan-Kudan deposit.</p

Ore-forming processes in the Khetri Copper Belt, western India:constraints from trace element chemistry of pyrite and C-O isotope composition of carbonates

The Khetri Copper Belt of the Aravalli-Delhi Fold Belt in western India hosts Cu (± Au ± Ag ± Co ± Fe ± REE ± U) mineralization that is likely of iron oxide-copper-gold (IOCG) type. The study on the Madan-Kudan deposit in this belt documents four vein types: Type-1 (pyrite ± chalcopyrite ± magnetite ± biotite ± scapolite ± amphibole ± chlorite), Type-2 (chalcopyrite-pyrrhotite-pyrite-magnetite-amphibole-chlorite), Type-3 (chalcopyrite-pyrrhotite-pyrite-dolomite-quartz), and Type-4 (chalcopyrite-pyrrhotite-biotite). Pyrite is grouped on texture and major and trace element chemistry into Pyrite-1A, Pyrite-1B, Pyrite-1C (Type-1 veins), Pyrite-2 (Type-2 veins), Pyrite-3A, and Pyrite-3B (Type-3 veins). This sequence documents changing fluid composition and suggests that sulfide mineralization was associated with Na-Ca-K alteration (Type-1 and Type-2 veins), carbonate alteration (Type-3 veins), and K-Fe-Mg alteration (Type-4 veins). The C and O isotope composition of dolomite from Type-3 veins suggests that the ore fluid contained mantle-derived carbon (possibly carbonatite-related) and mixed with an isotopically heavier fluid or exchanged isotopes with crustal rocks. A strong positive correlation between Au and Cu is interpreted to reflect their “coupling” in the pyrite structure. In contrast, Pb, Zn, Bi, and Ag are present in mineral inclusions. Intragrain Fe, Co, As, and Ni variability in pyrite suggests that replacement by coupled dissolution-precipitation and formation of overgrowths were important. Pyrite-1A has high Co (up to 3.3 wt%) and Co/Ni ratios (500 to 16,000) that have not been reported elsewhere. The Co/Ni ratios of KCB pyrite are similar to those from iron oxide-apatite and other IOCG deposits, although the latter do not have a characteristic Co/Ni ratio but consistently have high Co concentrations (up to 1 wt% or more).</p