Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India

Geochemistry of the Meso- to Neoproterozoic basic-acid rocks from Hunan Province, South China: implications for the evolution of the western Jiangnan orogen

Abstract: The formation and evolution of the Jiangnan orogen at the southeastern margin of the Yangtze Block, South China, are an important and debatable topic. The Meso- to Neoproterozoic basic–acid rocks from Hunan Province record the history of the western Jiangnan orogen in the area. The Mesoproterozoic basalts and diabases from Nanqiao are the typical N-MORB, having very low K2O, low incompatible HFSE and REE, and depleted ɛNd (T) values (+6.86 to +8.98). They may represent the fragments of an obducted oceanic crust or the relicts of the oceanic crust in a “forearc basin” along an ancient subduction zone, which provides new evidence for the existence of the Jiuling Old Island Arc. The Mesoproterozoic komatiitic basalts from Yiyang are high in Al2O3/TiO2, MgO, Ni and Cr, and are depleted in Nb and Ti. These plume-derived magmas are associated with island-arc tholeiites and exhibit the geochemical characteristics of the arc magma, suggesting that the Mesoproterozoic komatiitic basalts might be the products of the plume–arc interaction. The Neoproterozoic andesitic rocks from Baolinchong, with strong depletions of Nb, Ti and enrichment of LILEs, are of typical island–arc volcanic origin. The Neoproterozoic granites from northeastern Hunan are strongly peraluminous (SP) granites, with high ASI (>1.1) and high CaO/Na2O ratio (>0.3), suggesting that they might be derived from the partial melting of the psammitic sources in the Mesoproterozoic Lengjiaxi Group. The field geological, chronological, petrographical, geochemical, and isotopic features of these granites indicate that they are the products of post-collisional magmatism related to the breakoff of subducted slab, similar to the andesitic rocks from Baolinchong. The Neoproterozoic basic rocks from the western and central Hunan have the geochemical signatures of rifting basalts, with normalized patterns similar to ocean island basalts (OIB). Their occurrence in intraplate extensional environments is considered as indicators of the rifting of the area. Both the distribution style and field geological evidence of these Neoproterozoic basic rocks are not in line with a mantle plume model. Based on geochemical and petrological studies of the Meso- to Neoproterozoic basic–acid rocks, a preliminary model for the formation and evolution history of the western Jiangnan orogen in the area was put forward.

Generation and evolution of siliceous high magnesium basaltic magmas in the formation of the Permian Huangshandong intrusion (Xinjiang, NW China)

Abstract: Numerous Permian orthopyroxene-rich mafic–ultramafic intrusions in the southern margin of the Central Asian Orogenic Belt (CAOB) have been emplaced in a post-orogenic environment. The Huangshandong mafic–ultramafic intrusion is the largest in the eastern Tianshan Orogenic Belt of the CAOB and consists of a layered unit intruded by a massive unit. The layered unit consists of lherzolite, troctolite, olivine gabbro, hornblende gabbro, gabbrodiorite and diorite, whereas the massive unit is made of gabbronorite and olivine gabbronorite. Olivine from the layered unit has Fo values of 75 to 84 and contains low NiO and MnO. Rocks from this unit have initial 87Sr/86Sr ratios (0.7031–0.7043) and eNd values (+ 3.4–+ 7.1) while plagioclase has initial 87Sr/86Sr ratios ranging from 0.70010 to 0.71569. Olivine from the massive unit has relatively low Fo values of 59 to 70 and high NiO and MnO. Rocks from this unit have relatively low and uniform initial 87Sr/86Sr ratios (0.7030) and high eNd values (+ 6.0–+ 7.5) and contain plagioclase with relatively low initial 87Sr/86Sr ratios (0.70042 to 0.70520). Olivines from both units have low CaO and Cr2O3. Both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous, depleted mantle source. Magma forming the layered unit experienced earlier removal of sulfides before emplacement, with the earlier sulfide-saturation caused by the addition of crustal material. The magma that formed the massive unit has undergone fractional crystallization of olivine ± Cr-spinel. Siliceous high magnesium basaltic magmatism, produced by melting of a hydrous depleted mantle source, may be an important feature of orogenic belts.

The geochemistry of ultramafic to mafic volcanics from the belingwe greenstone belt, Zimbabwe: Magmatism in an archean continental large igneous province

Abstract: The evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks. Four volcanic types (komatiite, komatiitic basalt, D-basalt and E-basalt) are distinguished in the 2� 7Ga Ngezi volcanic sequence using a combination of petrography and geochemistry. The komatiites and D-basalts are rocks in which isotopic systems and trace elements are depleted. Chemical variations in komatiites and D-basalts can be explained by fractional crystallization from the parental komatiite. In contrast, komatiitic basalts and E-basalts are siliceous and display enriched isotopic and trace element compositions. Their chemical trends are best explained by assimilation with fractional crystallization (AFC) from the primary komatiite. AFC calculations indicate that the komatiitic basalts and E-basalts are derived from komatiites contaminated with � 20% and � 30% crustal material, respectively. The volcanic stratigraphy of the Ngezi sequence, which is based on field relationships and the trace element compositions of relict clinopyroxenes, shows that the least contaminated komatiite lies between highly contaminated komatiitic basalt flows, and has limited exposure near the base of the succession. Above these flows, D- and E-basalts alternate. The komatiite appears to have erupted on the surface only in the early stages, when plume activity was high. As activity decreased with time, komatiite magmas may have stagnated to form magma chambers within the continental crust. Subsequent komatiitic magmas underwent fractional crystallization and were contaminated with crust to form D-basalts or E-basalts.

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

Abstract: Siliceous high-Mg basalt (SHMB) is a rare rock type that occurs mainly at or near the Archean–Proterozoic boundary. It shares some geochemical similarities with Phanerozoic boninites, but there is a clear distinction. Whether the petrogenesis of SHMB resembles that of Phanerozoic boninites or is related to the komatiitic magmatism is controversial. Neoarchean SHMBs are identified for the first time from the Taishan granite–greenstone terrane within the Eastern Block of the North China Craton (NCC). Zircon U–Pb dating indicates that they were emplaced at ∼2.54 Ga, contemporaneous with the generation of sanukitoids and adakitic rocks, but later than the eruption of the Late Archean (2.71 Ga) komatiites and komatiitic basalts in the area. The high MgO (>8%), high SiO2 (>52%), and Al2O3/TiO2 ratio (12.4–45.0), together with low TiO2 (<0.5%) and HFSE contents and strong enrichment in LREE and LILE in the Taishan SHMBs are comparable with typical Phanerozoic boninites, except for distinct HREE depletion, lack of U-shaped REE patterns and conspicuous positive Zr anomalies. In conjunction with their more depleted Nd isotopic characteristics (ɛNd(t = 2.54 Ga) = +4.42 to +1.05) relative to Late Archean komatiites in the region, it suggests that these SHMBs were derived from partial melting of refractory depleted mantle which experienced earlier basalt extraction and was subsequently enriched in LILE and LREE by subduction-related metasomatization, rather than the products of assimilation–fractional crystallization (AFC) of komatiitic magma. A slab-derived adakitic melt was likely the metasomatizing agent, along with minor aqueous fluids released from the subducting oceanic slab. In combination with regional studies, the generation of these magmas was probably related to slab rollback, which is ascribed to the arrival of an oceanic plateau and/or residual thickened lithospheric keel at the subduction zone at that time. This mechanism might have played a crucial role in the formation of Archean granite–greenstone belts and was an important factor in continental crustal growth, particularly during the Late Archean.

Precambrian mafic magmatism in the Bastar craton, central India

Abstract: The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Abstract: 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.

The continental crust: Its composition and evolution

Abstract: This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.

The composition of the Earth

Abstract: Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements. The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth. The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga). The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition. The Earth has a bulk Fe Al of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of Fe Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.

A Guide to the Chemical Classification of the Common Volcanic Rocks

Abstract: A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.