In recent years, application of statistical analysis to a sufficiently large geochemical database has become more and more popular to reveal the onset and evolution of plate tectonics as a consequence of the development of computer science and storage technology. Here, we introduce a robust statistical method to process a filtered geochemical dataset including predominantly continental basaltic rocks through Earth's history. The results show that the average Sr concentrations gradually increased from 3.8 to 0 Ga, indicating a progressively growing depth of magma source as a result of secular cooling of the mantle. However, the average La/Yb and Sm/Yb ratios only started to increase at ca . 3.0 Ga. Considering the La–Yb and Sm–Yb fractionations are commonly attributed to the residual garnet crystallization, we interpreted that the geotherm of the magma source of continental basaltic rocks before 3.0 Ga might be higher than the stability limit of garnet. As such, there were little or no garnets in the residue during partial melting of the upwelling asthenospheric mantle or mantle plumes before 3.0 Ga, which also implied a higher continental geotherm during that time. Such a high geotherm could not support the formation of contemporaneous high‐pressure or medium‐pressure TTGs (tonalite–trondhjemite–granodiorite) by vertical tectonic models associated with upwelling mantle heating or delaminated lower crust melting in the early Archean. We therefore conclude that the plate tectonics had occurred by ca . 3.8 Ga in a hot subduction style and contributed to the formations of high‐ and medium‐pressure TTGs subsequently.
Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere.International Ocean Discovery Program Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel.Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 19 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access materials from this zone.Active conduits, resulting from fractures in the forearc, are prompted by along-and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes.Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth.Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc.Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid.Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel.These processes have likely been active for tens of millions of years at the Mariana forearc and for billions of years on Earth.
Compositional differences between continental and oceanic arc volcanic rocks have long been recognized; however, our understanding of them is incomplete. This study presents a comprehensive geochemical comparison, including major, trace and Sr–Nd–Pb isotope compositions, between global continental and oceanic arc volcanic rocks. Our data compilation reveals that volcanic rocks from a single oceanic arc occupy basaltic to basaltic–andesitic average lithochemical compositions, except for volcanic rocks from the Lesser Antilles arc, which occupies an andesitic average lithochemical composition, similar to the bulk composition of volcanic rocks from single continental arcs. The composition of the average continental arc volcanic rocks, the average Lesser Antilles arc volcanic rocks and the average oceanic arc volcanic rocks are different in many ways, which span the entire compositional range from basalts to rhyolites, and are reflected in major, trace element and Sr–Nd–Pb isotope compositions. Qualitative geochemical analyses and semi‐quantitative modelling suggest that the compositional differences in mafic arc volcanic rocks can be accounted for by differences in source mixing between mantle wedge peridotite and slab‐derived crustal components. Slab melts‐peridotite source mixing governs most of the lithochemical features of the continental arc and the Lesser Antilles arc mafic volcanic rocks, which is especially true for Sr–Nd–Pb isotopes. On the other hand, such source mixing occurred to a lesser extent in the petrogenesis of oceanic arc mafic volcanic rocks. Magma evolution processes such as fractional crystallization may further modify the lithochemical composition of the arc volcanic rocks, especially the felsic ones, but this is not the governing factor.
The mechanism of crustal recycling in subduction zones has been a heated debate, and Mg–Fe isotopes may provide new constraints for this debate. This study reported the Fe–Mg isotope data for mafic plutonic rocks from the eastern and central Gangdese arc and their associated trench sediments in southern Tibet. The δ26Mg (–0.32 to –0.20‰) and δ56Fe (0.04 to 0.12‰) values of the eastern Gangdese arc rocks show negative and positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but positive and negative correlations with εNd(t) and εHf(t) values, respectively. The Mg and Fe isotopic compositions (δ26Mg = –0.28 to –0.15‰; δ56Fe = 0.02 to 0.12‰) of the central Gangdese arc rocks are comparable with the eastern ones, but they are not covariant with Sr–Pb–Nd–Hf isotopes. More importantly, the Fe–Mg isotopes for most of the arc rocks fall in between local trench sediments (δ26Mg = –0.61 to –0.30‰; δ56Fe = 0.00 to 0.17‰) and the normal mantle. Integrated qualitative analyses and quantitative simulations suggest that while the Mg–Fe isotope variations in the eastern Gangdese arc rocks revealed the important role of source mixing between sediment-derived melts and peridotite, their variations in the central Gangdese arc rocks reflected the controlling effects of source mixing between carbonated serpentinite-derived Mg-rich fluid and peridotite and source melting. The good covariant relationships between Mg–Fe isotope and traditional geochemical tracers provide further evidence for the recycling of crustal materials in subduction zones via various types of slab-derived fluids and melts.
Rocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean 206Pb/238U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon εHf(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants.
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