Proterozoic Subduction-Related and Continental Rift-Zone Mafic Magmas from the Eastern Ghats Belt, SE India: Geochemical Characteristics and Mantle Sources
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Understanding the origin and growth of continental crust is a fundamental problem in geological sciences. Two distinct ways in which the continental crust grows include horizontal (subduction) and vertical (plume/extension) accretions. As the mantle reservoirs in these two tectonic settings are generated and/or modified by contrasting processes, the erupted melts offer clues on the nature of these divergent mantle sources. Trace element geochemistry is a robust tool to quantitatively model the mantle sources, melting mechanisms and relative roles of mantle and crust in the petrogenesis of magmatic rocks, which ultimately lead us to unravel the origin of continental crust. The present study portrays growth of the continental crust in the Proterozoic Eastern Ghats Belt, SE India. Mafic magmas within the Palaeoproterozoic Kondapalli-Kandra region illustrate subductionrelated island arc basalt-type geochemical signatures whereas alkali basaltic magmas in the Mesoproterozoic Prakasam continental rift-zone display ocean island basalt-type characteristics. Calculated mantle sources for subduction-zone and rift-related magmas display distinctly different geochemical traits. Mesoproterozoic gabbroic magmas in the Prakasam rift-zone exhibit geochemical signatures akin to the subduction-related mafic melts. This dichotomy of continental crust produced by intra-plate processes exhibiting plate-margin signatures advocates that we possibly have overestimated the proportion of continental crust generated above subduction zones.Keywords:
Continental Margin
Mantle plume
Petrogenesis
Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.
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Author(s): Hagen-Peter, Graham Adrian | Advisor(s): Cottle, John M | Abstract: The process of subduction has significant influence on the geochemical evolution of the crust and mantle, and subduction-related magmatism may have been an important mechanism for the growth of continental crust over time. Extensive exposure of the mid-crustal levels of an ancient, exhumed continental arc in the Transantarctic Mountains provides an exceptional opportunity to study the metamorphic and magmatic processes associated with an archetypal subduction zone. The belt of multiply deformed metamorphic rocks and granitoid batholiths records convergence and subduction of paleo-Pacific oceanic lithosphere beneath East Antarctica during the Neoproterozoic–Paleozoic Ross orogeny. These rocks are especially well exposed in the southern Victoria Land (sVL) segment of the Transantarctic Mountains––the largest ice-free area of Antarctica. The metamorphic and igneous rocks in sVL provide insights into the early stages of convergent-margin tectonism, the compositional diversity of subduction-related magmatism, and the relative roles of crustal growth and recycling in continental arcs. In Chapter 1 of this dissertation, garnet Lu-Hf and monazite U-Pb geochronology—combined with petrography, mineral chemistry, and thermobarometry—reveal a Barrovian-style metamorphic history that predated the dominant phase of magmatism in sVL. The geochronology data from this study provide one of the oldest records of tectonism along the Ross orogen. The results are consistent with a tectonic model that involves shortening across the margin of East Antarctica prior to the major phase of subduction-related magmatism. Chapter 2 explores the age and magma sources of a large subduction-related igneous complex in the Dry Valleys area. Zircon U-Pb geochronology demonstrates that the period of magmatism in the Dry Valleys was relatively short-lived compared to other segments of the Ross orogen. Whole-rock geochemistry and Hf isotopes in zircon reveal the assimilation of ancient crust during the differentiation of juvenile magmas that were likely derived from an enriched sub-continental lithospheric mantle source. A compilation of Nd and Sr isotope data from granitoids from along the Ross orogen suggest that enriched lithospheric mantle may have been a common juvenile magma source along the arc. In Chapter 3, a comprehensive geochemical, geochronologic, and isotopic investigation of the magmatism in sVL explores the conspicuous occurrence of alkaline silicate rocks and carbonatites—most commonly associated with intraplate and continental-rift magmatism—within the continental arc. The alkaline magmatism was partially contemporaneous with the emplacement of large sub-alkaline igneous complexes in adjacent segments of the arc. The isotopic and trace element composition of the alkaline and subalkaline rocks suggests derivation from geochemically enriched sources—potentially metasomatized sub-continental lithospheric mantle. Despite enriched isotopic and trace element compositions that broadly resemble recycled continental crust, binary mixing and assimilation-fractional crystallization models indicate that crustal growth may have been dominant over crustal reworking in the sVL magmatism.
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Granitoids are the main component of the continental crust, and the key problem for understanding the structure, composition and evolution of the continental lithosphere. Late Mesozoic calc alkaline magmatism is well developed in the coastal area of southeastern China, and is similar to the magmatism developed in the Basin and Range of North American. During the previous studies on the granites in the coastal area of southeastern China, however, little attention has been paid to the temporally and spatially coexisted A type granite, gabbroic stocks or dikes, as well as the bimodal volcanic rocks (basalt rhyolite composite lavas). The authors have been studying the granites in the coastal area for many years and noticed recently that this region experienced extensive crust mantle interaction at the crust mantle boundary, and underplating was the important process which involved in the chemical and thermal contributions from the mantle to the crust. The petrography, geochemistry and isotope studies for the gabbroic granulite xenoliths from Qilin basaltic pipe in the same region, and integration with the regional geophysical data indicate that gabbroic granulites were formed by underplating of basaltic magmas at the bottom of the crust at ca 112 Ma. This is consistent with the extensive granitic magmatism in the late Mesozoic time along the coastal area of southeastern China. Therefore, underplating plays an important part in the evolution of the continental crust of southeastern China. Most of the granites formed in the earlier stage of late Mesozoic time in southeastern China are S type granites. They might be generated from the partial melting of the deeper crust material under the compressive tectonic setting. While the extensive I type and A type granitic magmatism in the coastal area of southeast China is closely related with back arc extension, lithosphere thinning and asthenosphere upwelling, which may have been induced by the earlier stage subduction of the Pacific plate towards the Eurasian continent. In fact, the late Mesozoic magmatisms along the coastal area have an essentially extension related bimodal character, and apparently, the genesis of granites in the coastal area of southeastern China is directly related with the basaltic magma underplating. The suture age of 100～110 Ma of southeast China may represent the transition age of tectono magmatism from “compression crust thicking continental crust re melting” to “extension lithosphere thinning bimodal magmatism”.
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This paper discusses the ocean—continent transition of the magmatic arc and its processes in terms of the igneous petrotectonic assemblage,the geologic characteristics,and the arc maturity. The major igneous petrotectonic assemblages characterized the oceanic subduction may be tonalite—trondhjemite—granodiorite( TTG),high-Mg andesitic,Mg andesitic,adakitic and Nb-enriched arc basaltic assemblages. Based on the disposition of the petrotectonic assemblages,four possible models of oceanic subducted crust—mantle structure are discussed:( 1) the warm young subducted oceanic crust and the above-situated cool mantle-wedge lithosphere;( 2) both the cool older subducted oceanic crust and the mantle-wedge lithosphere;( 3) the cool older subducted oceanic crust and the above-situated warm mantle-wedge asthenosphere;( 4) both the warm oceanic crust and the mantle-wedge asthenosphere. The tectonic indications of the arc magmatic front as the structure markers and the special compositional polarity are also discussed. A model of two-layered arc crust is suggested,the lower arc crust may be composed of the mafic granulites and amphibolites,and the upper crust consists of the felsic TTG gneisses.The petrologic structure of the arc crust may have a typical transitional character between the ocean crust and the continental crust or may be regarded as the first stage of the continental crust,the juvenile continental crust. The magmatic arc and the transitional character of the arc crust are regarded as the most important record for the ocean—continent transitional belts( or the accretional orogenic belts) derived by the oceanic subduction.
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Research Article| June 01, 2010 Do Cenozoic analogues support a plate tectonic origin for Earth's earliest continental crust? Alan R. Hastie; Alan R. Hastie 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Andrew C. Kerr; Andrew C. Kerr 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Iain McDonald; Iain McDonald 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Simon F. Mitchell; Simon F. Mitchell 2Department of Geography and Geology, University of the West Indies, Mona, Kingston 7, Jamaica Search for other works by this author on: GSW Google Scholar Julian A. Pearce; Julian A. Pearce 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Martin Wolstencroft; Martin Wolstencroft 1School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK Search for other works by this author on: GSW Google Scholar Ian L. Millar Ian L. Millar 3NERC Isotope Geoscience Laboratories, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (6): 495–498. https://doi.org/10.1130/G30778.1 Article history received: 14 Oct 2009 rev-recd: 14 Dec 2009 accepted: 24 Dec 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alan R. Hastie, Andrew C. Kerr, Iain McDonald, Simon F. Mitchell, Julian A. Pearce, Martin Wolstencroft, Ian L. Millar; Do Cenozoic analogues support a plate tectonic origin for Earth's earliest continental crust?. Geology 2010;; 38 (6): 495–498. doi: https://doi.org/10.1130/G30778.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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The East African Rift System (EARS) and the West Antarctic Rift System (WARS) are two of the largest continental rift systems on Earth, but the processes governing rift dynamics remain controversial. The large volume and distinct chemistry of Cenozoic rift-related lavas, combined with geophysical evidence for low-density mantle underlying both rifts, have traditionally been interpreted as evidence for anomalously hot mantle plumes actively rising from the deep mantle beneath both regions. However, in light of increasing evidence that these mantle structures may also be chemical in nature, alternative explanations highlight the role of heterogeneous, easily-fusible mantle components in driving magma genesis in the absence of significant thermal anomalies. These heterogeneous mantle domains may be linked to the complex tectonic histories of continents, which often involve multiple stages of accretion and associated recycling of materials between the crust and the mantle. In this dissertation, detailed chemical investigations of rift-related volcanic rocks provide critical new insight into the nature of the mantle underlying the two active rifts. In particular, the role of volatiles such as water and carbon dioxide is highlighted due to the ability of these components to enhance mantle melting during rift-related decompression. In Chapter II, the first geochemical information from submarine lavas in the Ross Embayment of West Antarctica are reported alongside subaerial lavas from islands and mainland localities, which together provide evidence that volatilized, recycled mantle domains generated during ancient long-lived subduction along the paleo-Pacific margin of Gondwana are key components in the temporally evolving source of Cenozoic magmas (Aviado et al., 2015). In Chapter III, the first rift-wide study of magmatic volatiles recorded in olivine-hosted melt inclusions confirms that the West Antarctic mantle is enriched in water and carbon dioxide on a wide scale, and links the production of hydrated and carbonated components to subduction-related metasomatic processes. These results provide a compelling link between continental assembly, the production of easily fusible, heterogeneous chemical domains in the sub-continental lithospheric mantle (SCLM) and upper mantle, and rifting plus associated magmatism. These links are further explored in Chapter IV, in which the trace element and radiogenic isotope (Sr-Nd-Hf-Pb) systematics of mantle xenoliths from the East African Rift System (EARS) illustrate that the SCLM bears witness to a complex history involving Proterozoic melt depletion events, Pan African continental assembly, late-stage metasomatism, and plume impingement. These results demonstrate that SCLM serves as an important long-lived host of heterogeneous recycled mantle domains that are sampled throughout multiple episodes of convergence and breakup. Collectively, these chapters suggest that ongoing rifting and magmatism in the WARS and the EARS are in part tied to shallow mantle processes…
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Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type ( eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.
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Tectonic models have postulated that subduction of a flat‐lying oceanic plate was intimately linked with mid‐Eocene magmatism in the northwestern United States and have assumed that this linkage is expressed in the geochemical characteristics of igneous rocks. Geochemical data from the Crazy Mountains in south central Montana can be interpreted as indicating the presence of a subduction‐related chemical component. Because radiogenic isotopic data for these rocks restrict the major chemical events in their mantle source to the Late Cretaceous‐Early Tertiary (50–100 Ma) and the mid‐Proterozoic (1.6–1.8 Ga), subduction during one or both of these periods is the most plausible explanation for the origin of the arc‐like geochemical features. Assessment of geologic and tectonic constraints, however, indicates that Late Cretaceous‐Early Tertiary subduction cannot explain the regional distribution, the contemporaneity, and the compositional range of the Eocene igneous rocks. The Eocene magmatic event does not show interpretable age progressions, as most subduction‐related models suggest. There is considerable geographic overlap in the chemical characteristics of the Eocene igneous rocks, so that chemical zoning cannot be demonstrated south of the U.S.‐Canada border. The zone of igneous activity was diffuse and discontinuous, unlike most subduction‐related magmatic arcs. The presence of thick lithospheric mantle beneath the Archean Wyoming craton presents difficulties for models that involve subduction of a flat‐lying slab. The lithospheric mantle probably controlled the depth of subduction and probably forced the slab to depths greater than those of plausible source regions for subalkalic, mantle‐derived basalts. Flat slab subduction models cannot account for the thermal inputs required for extensive magmatism. Subduction‐related models fail to explain the CO 2 ‐rich character of alkalic magmatism in central Montana. The arc‐like geochemical patterns in the Crazy Mountains samples could result either from subduction unrelated to Late Cretaceous‐Early Tertiary events, or from mantle geochemical processes that are unrelated to subduction. If geochemical processes unrelated to subduction are involved, trace element discrimination diagrams are misleading and are inadequate criteria for interpreting the tectonic environment. The evidence for mid‐Proterozoic chemical modification of the mantle beneath the Wyoming craton suggests that the arc‐like character could be inherited from subduction events between 1.6 and 1.9 Ga, but a specific mid‐Proterozoic subduction geometry cannot be identified. Alternative tectonic models for Eocene magmatism can be built around plausible heating or decompression mechanisms for regional magma generation. The advection of a thermal anomaly by mantle upwelling, or regional uplift related to the thickening‐flexure‐rebound cycle in the Cordillera to the west could have triggered the regional Eocene magmatic event.
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