Zircon U-Pb geochronology and elemental and Sr–Nd isotope geochemistry of Permian mafic rocks in the Funing area, SW China

Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO₂ and P₂O₅ higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial ⁸⁷Sr/⁸⁶Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ∼260 Ma.     

Compositional variations of chromiferous spinel in Mg-rich rocks of the Deccan Traps,India

Composition of chromiferous spinel included in olivines of Mg-rich basalts and gabbros of the Deccan Traps (Gujarat and Western Ghats) are reported here. They vary from Al-rich compositions [Al₂O₃ = 53wt.%; Cr#, 100Cr/(Cr + Al) = 12] to Cr-rich compositions [Cr₂O₃ = 51wt.%; Cr# = 84], and from Cr-Al rich compositions towards Cr-rich Ti-magnetite (TiO₂ up to 23 wt.%, ulvöspinel up to 67mol.%). The Mg# [100Mg/(Mg + Fe²⁺)] of spinel decreases from 81 to nearly zero. The highest Cr# has been found in the Bushe Fm., Thakurvadi Fm., and some high-Ti basalts of the Pavagadh section, whereas some of the low-Ti basalts of Saurashtra have Al-rich compositions typical of spinels found in mid-ocean ridge basalts. The chemical composition of the Deccan Trap spinels is completely different compared to that observed in mantle spinel suites, with very few exceptions. The decreasing Al and increasing Fe and Ti of spinel seems to be mainly the result of decrease of Mg in the locally coexisting melts and favourable cationic substitutions in the lattice. There is barely any evidence of general relationships between the composition of the Deccan spinels and inferred mantle sources of the host magmas. Pyroxene inclusions in spinels may witness a high-pressure stage of crystallization, but the possibility of non-equilibrium crystallization, or even magma mixing, cannot be ruled out. Overall, the compositional ranges of chromiferous spinel in the Deccan Traps closely match those observed in the other Large Igneous Provinces having mafic/ultramafic intrusions and mafic magma compositions (e.g., Siberian Traps, Karoo, Emeishan).     

Low-Ti melts from the southeastern Siberian Traps Large Igneous Province: Evidence for a water-rich mantle source?

Siberian Traps Large Igneous Province (STLIP) is one of the most voluminous volcanic provinces on Earth. The dominant erupted rocks are low-Ti basalts, which make up 80% by volume of the classical Noril’sk lava sequence. In the west Siberian basin and Maymecha-Kotuy area, the low-Ti basalts make up about 99% and 50% by volume, respectively. Dolerite sills in the Angara-Taseevskaya Syncline at the southeastern STLIP exhibit trace element patterns and Sr isotope ratios typical of the low-Ti basalts of the Noril’sk sequence. The most Mg-rich (MgO 9.5–11 wt%) and hence least differentiated dolerites are characterized by trace element patterns with Ta-Nb depletion, low Ce/Pb and high Sr/Pr. These trace element features are similar to water-saturated, mantle wedge-derived island arc basalts. These imply an important role of subduction fluid-derived trace elements in the source of melting beneath the Angara-Taseevskaya Syncline and other regions of the STLIP. Less magnesium rocks (MgO 3.8–6.1 wt%) with less prominent Ta-Nb depletion, higher Ce/Pb and lower Sr/Pr could be produced via olivine-plagioclase fractionation of primary high-magnesium melts.     

Geochemistry and petrogenesis of sodic and potassic mafic alkaline rocks in the Deccan Volcanic Province, Mumbai Area (India)

Summary Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres, tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12–2 wt%). The lamprophyres have high content of incompatible elements (e.g., TiO₂ > 3.8 wt%, Nb > 130 ppm, Zr > 380 ppm, Ba > 1200 ppm), and relatively high initial (at 65 Ma) ¹⁴³Nd/¹⁴⁴Nd (0.5128) and low ⁸⁷Sr/⁸⁶Sr (0.7038–0.7042). They are likely to be small-degree melts (2–3%) of volatile- and incompatible element-enriched mantle sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks have highly porphyritic textures (up to 57% phenocrysts of diopside ± olivine), and anomalously low contents of incompatible elements (e.g., TiO₂ < 1.3 wt%, Nb < 24 ppm, Zr < 100 ppm) indicating that they could not represent liquid compositions. Moreover, their very low initial ¹⁴³Nd/¹⁴⁴Nd ratios (0.5116–0.5120), at ⁸⁷Sr/⁸⁶Sr = 0.7045–0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that could be identified as “Lewisian-type” lower crust and/or enriched mantle. The melting episode that generated these alkaline rocks likely occurred close to the base of the ca. 100 km-thick Indian lithosphere, very shortly after the main eruption of the Deccan tholeiites. Received January 14, 2000; revised version accepted September 28, 2001     

Highly heterogeneous Precambrian basement under the central Deccan Traps, India: Direct evidence from xenoliths in dykes

Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured ⁸⁷Sr/⁸⁶Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day ⁸⁷Sr/⁸⁶Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions.     

Geochemistry and Petrogenesis of the Proterozoic Bandal Mafic Rocks,Himachal Pradesh,NW Himalaya

The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO₂ (> 2 wt%), Ti/Y, Ti/Zr, TiO₂/K₂O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO₂ (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km² in the Lesser Himalaya provides an evidence of plume activity in the region.     

Rift-associated ultramafic lamprophyre (damtjernite) from the middle part of the Lower Cretaceous (125 Ma) succession of Kutch,northwestern India: Tectonomagmatic implications

Mineralogical, geochemical and isotopic (Sr and Nd) studies on the recently reported ca. 124 Ma ‘anorogenic lamproite’ dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. We propose a new classification for the dyke as a damtjernite (ultramafic lamprophyre; UML) based on its porphyritic-panidiomorphic texture, abundance of phlogopite, presence of nepheline in the groundmass, and the composition of liquidus phases such as olivine, phlogopite, magnetite, and clinopyroxene (diopside). The Palanpur UML is primitive (Mg# = 74–77), silica-undersaturated (SiO₂ <40 wt.%), potassic to slightly sodic in nature, and is strikingly similar to the ∼69 Ma UML dykes and sills of the Tethyan Indus suture zone, which are considered as the earliest yet known manifestations of the Deccan Large Igneous Province (LIP). Bulk-rock (⁸⁷Sr/⁸⁶Sr)_i (0.70460–0.70461) and ɛ_Nd(t) (+2.56 to −0.69) of the Palanpur UML signify derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. This is further attested to by the high incompatible trace element ratios (viz., La/Ba, Nb/U, Nb/La and Ta/Yb) that are typical of plume-type magmas. However, the Neoproterozoic T_DM depleted mantle Nd model ages (∼655–919 Ma) also necessitate some involvement of a lithospheric mantle component in its genesis. High bulk-rock Fe₂O₃^t and TiO₂ contents require the involvement of a fertile peridotitic mantle source, whereas high La/Yb (60–80) implies a control by residual garnet. Higher Rb/Sr and lower Ba/Rb suggest phlogopite as a residual phase and high Nb and lower La/Sm favour carbonatite, rather than silicate melt as metasomatising agent. Low degrees of partial melting of a primitive garnet lherzolite mantle can account for the observed REE patterns in the Palanpur UML. The Palanpur UML shares a temporal similarity to the Kerguelen plume-derived Rajmahal basalts and associated alkaline rocks from the eastern India. The tectonomagmatic significance of its emplacement during the mid-Cretaceous vis-à-vis various models involving the timing of eruption of the Deccan and the Rahmahal Traps and the rifting in the Kutch basin induced by far-field plate reorganization is evaluated.     

Composition and age of tertiary sills and dykes, Jameson Land Basin, East Greenland: relation to regional flood volcanism

The Paleozoic–Mesozoic Jameson Land Basin (East Greenland) is intruded by a sill complex and by a swarm of ESE trending dykes. Together with dykes of the inner Scoresby Sund fjord, they form a regional Early Tertiary intrusive complex located 200–400 km inland of the East Greenland rifted continental margin. Most of the intrusive rocks in the Jameson Land Basin are geochemically coherent and consist of evolved plagioclase–augite–olivine saturated, uncontaminated high-Ti basalt with 48.5–50.2 wt.% SiO₂, 2.2–3.2 wt.% TiO₂, 5.1–7.4 wt.% MgO, 9–17 ppm Nb and La/Yb_N=2.8–3.6. Minor tholeiitic rock types are: (a) low-Ti basalt (49.7 wt.% SiO₂, 1.7 wt.% TiO₂, 6.8 wt.% MgO, 2.6 ppm Nb and La/Yb_N=0.5) akin to oceanic basalts; (b) very-high-Ti basalt (48.6 wt.% SiO₂, 4.1 wt.% TiO₂, 5.1 wt.% MgO and 21 ppm Nb); and (c) plagioclase ultraphyric basalt. The tholeiitic dolerites are cut by alkali basalt (43.7–47.3 wt.% SiO₂, 4.1–5.1 wt.% TiO₂, 4.9–6.2 wt.% MgO, 29–46 ppm Nb and La/Yb_N=16–17) sills and dykes.Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from 6–10% melting of approximately equal proportions of garnet- and spinel-bearing mantle of slightly depleted composition beneath thick continental lithosphere. Conversely, dolerite intrusions and flood basalts of similar compositional kindred from adjacent but more rift-proximal occurrences in Northeast Greenland formed from shallower melting of dominantly spinel-bearing mantle beneath extended and thinned continental lithosphere. These variations in lithospheric thickness suggest the continent–ocean transition of the East Greenland rifted volcanic margin is sharp and narrow.⁴⁰Ar–³⁹Ar dating and paleomagnetism show that the high-Ti dolerites were emplaced at 53–52 Ma (most likely during C23r) and hence surprisingly postdate the main flood volcanism by 2–5 Ma and the inception of seafloor spreading between Greenland and Europe by 1–2 Ma. The formation of tholeiitic and alkaline magmas emplaced into the Jameson Land Basin corroborates to the importance of post-breakup magmatism along the East Greenland volcanic rifted margin. Upwelling of the ancestral Iceland mantle plume under central Greenland at 53–52 Ma (rather than under the active rift), perhaps accompanied by a failed attempt to shift the rift zone westward towards the plume axis, may have triggered post-breakup continental magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments.     

Geochemistry of pillow basalts from Bompoka,Andaman–Nicobar islands,Bay of Bengal,India

Pillow lavas in Bompoka island of the Andaman–Nicobar islands, forming a part of Sunda–Burmese forearc, are composed of plagioclase and clinopyroxene microphenocrysts in a fine-grained ferruginous groundmass along with glass. They are also characterized by several quench plagioclase and clinopyroxene morphologies. Zr/TiO₂ versus Nb/Y relationship of these pillow lavas show that these are tholeiitic basalts in composition. These basalts have low MgO (5.19–6.12 wt%), Ni (84–118 ppm), and Cr (144–175 ppm) abundance and high FeO^(T)/MgO (1.71–1.92) ratios, reflecting their fractionated nature. In Th/Yb versus Nb/Yb and Ti/Yb versus Nb/Yb binary diagrams, they show N-MORB affinity. However, La/Nb–Y and Ce/Nb–Th/Nb relationships along with a slight LREE depleted (La_N/Yb_N = 0.75–0.82) pattern and high Ba/Zr (0.28–0.40) ratios and LILE (K, Rb, Ba, Sr and Th) enrichment relative to N-MORB, suggest their back-arc basin basalt affinity. It is inferred that these pillow basalts have been derived from a metasomatised N-MORB-like mantle source in a trench-distal (wider) back-arc basin, probably near the leading edge of the Eurasian continent during Early to Late Cretaceous times, prior to the currently active Andaman–Java subduction system.     

Constraints on the Mantle Sources of the Deccan Traps from the Petrology and Geochemistry of the Basalts of Gujarat State (Western India)

The late Cretaceous-early Tertiary flood basalts in the Gujaratarea of the northwestern Deccan Traps (Kathiawar peninsula,Pavagadh hills and Rajpipla) exhibit a wide range of compositions,from picrite basalts to rhyolites; moreover, the basaltic rockshave clearly distinct TiO₂ contents at any given degree of differentiationand strongly resemble the low-titanium and hightitanium basaltsfound in most of the Gondwana continental flood basalt (CFB)suites. Four magma groups are petrologically and geochemicallydistinguished: (1) A low-Ti group, characterized by rocks with varying SiO²saturation, and with TiO₂ <1•8 wt%, extremely low incompatibletrace element abundances, low Zr/ (av- 3•8), Ti/ V (av.27), and a very slight large ion lithophile element (LJLE) enrichmentover high field strength elements (HFSE). These rocks sharesome features with the Bushe Formation of the Western Ghatsfarther south, but have distinct geochemical characters, inparticular the strong depletion in most incompatible trace elements. (2) A high-Ti group, characterized by a more K-rich characterthan the low-Ti rocks, and with a strong enrichment in incompatibleelements, similar to average ocean island basalt (OIB), e.g.high TiO₂ (>1•8 wt% in picrites), Nb (>19 p.p.m.)Zr/ (av. 6•5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO₂ contents slightly lowerthan the high-Ti rocks at the same degree of evolution, andwith correspondingly lower incompatible trace element contentsand ratios, in particular K₂O, Nb, Ba and Zr/Y (av. 5•2). (4) A potassium-rich group (KT), broadly similar in geochemicalcharacter to the high-Ti group but showing more extreme K, Rband Ba enrichment (av. K₂0/Na₂0l; Ba/Y20). The most primitive low-Ti and high-Ti picrites, when correctedfor low-pressure olivine fractionation, show distinct major(and trace) element geochemistry, in particular for CaO/AI₂O₃,CaO/TiO₂ and Al₂O₃/TiO₂, and moderate but significant variationsin their SiO₂ and Fe₂O_st contents; these characteristics stronglysuggest the involvement of different mantle sources, more depletedfor the low-Ti picrites, and richer in cpxfor the high-Ti picrites,but with broadly the same pressures of equilibration (27–14kbar). This, in turn, suggests a strong lateral heterogeneityin the Gujarat Trap mantle. Low-Ti picrites and related differentiatesin Kathiawar are reported systematically for the first timehere, and suggest the existence of HFSE-depleted mantle in thenorthwestern Deccan Traps, with extension at least to the SeychellesIslands and to the area of the Bushe Formation near Bombay inthe pre-drift position, before the development of the CarlsbergRidge. The absence of correlations between LILE/HFSE ratiosand SiO₂ argues against crustal contamination processes actingon the low-Ti picrites, possibly owing to their probably rapiduprise to the surface. Consequently, the mantle region of thisrock group was probably re-enriched by small amounts of ULE-richmaterials. The substantially higher, trace element enrichmentof the least differentiated high-Ti picrites, relative to thebasalts of the Ambe-noli and Mahableshwar Formations of theWestern Ghats, testifies also to the presence of more incompatibleelement rich, OIB4ike mantle sources in northern and northwesternGujarat. These sources were geochemicaily similar to the present-dayReunion mantle sources. KEY WORDS: Deccan Traps; geochemistry; petrology; picrite basalts; western India ^*Corresponding author, e-mail: mellujo{at}ds.cued.unina.it     

Late Cretaceous mafic dykes in the Dharwar craton

Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (D _m =329°,I _m =−55°) and the corresponding palaeopole coordinates (λ _p = 34°S,L _p =108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.     

Volcanic Rocks from the Nejapa and Granada Cinder Cone Alignments, Nicaragua, Central America

Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al₂O_p,K₂O₁, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr₂O₃ concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and ⁸⁷Sr/₈₆Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO₂-K₂O concentrations. Thehigh-Ti basalts always have the lowest K₂O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low P_T and P_H2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and ⁸⁷Sr/⁸⁶Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand ⁸⁷Sr/⁸⁶Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on P_H2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K₂O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

Geochemistry of basic dykes from Betul-Jabalpur area in the Deccan volcanic province

Dykes exposed in the Betul-Jabalpur area, lie parallel to E-W trending Narmada-Son and Tapti lineaments in the Deccan volcanic province. These dykes show a variety of textural features and contain plagioclase (33–45%), clinopyroxene, olivine, magnetite and glass. These dykes are mainly basalt and basaltic andesite. Betul-Jabalpur and Tapti dykes show increase in sub-alkalis (K₂O+Na₂O) with the rise in SiO₂ values. Their data plots confine to the subalkalic array suggesting fractional crystallization as the dominant process. The high field strength elements in these dykes also show close correlation with the dykes south of the Tapti valley. Low concentration of Rb, Ba and V in Betul-Jabalpur dykes indicate that they are less contaminated than the other dykes of Deccan volcanic province. The large-scale chemical similarity in the major and trace elemental composition of the Betul-Jabalpur and south of Tapti valley dykes suggests their origin from a common magma type, possibly derived from the fractionation of isolated high gravity mafic-ultramafic igneous bodies positioned 6–8 km below the surface, trending parallel to the Narmada-Tapti rift zone.     

Geochemical Constraints on the Mantle Source of the Upper Permian Emeishan Continental Flood Basalts,Southwestern China

The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO₂, ranging from 43.6 to 52.1 wt%, Al₂O₃ from 5.0 to 12.6 wt%, and total alkali (K₂O + Na₂O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (¹⁴³Nd/¹⁴⁴Nd = 0.51229 -0.51276 and ⁸⁷Sr/⁸⁶Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume.

We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.     

Mineralogy,geochemistry and petrogenesis of the Khopoli mafic intrusion,Deccan Traps,India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNd_t?=??9.0 to ?12.7; ⁸⁷Sr/⁸⁶Sr?=?0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm³, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.     

Petrology,isotope characteristics,and K-Ar ages of the Maranhão,northern Brazil,Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this 700x250 km Maranhão province (5°–8°S) has low-Ti basalts (1.1 wt% TiO₂) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO₂) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb_(n) 4.2 and 8.8, where La_(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher ¹⁸O than the high-Ti basalts. Respectively, low- vs high-Ti: _Sr26–54 vs 15–18; ²⁰⁶Pb/²⁰⁴Pb=18.25–.78 vs 18.22–.24; and ¹⁸O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: _Nd–1.6 to –3.8 vs –2.1 to –3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb 7/4=4.6–11, and Pb 8/4=72–87; Serra Geral has Pb 7/4=10–13, and Pb 8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.     

Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province

Rocks of two distinctly different magma series are found in a ∼4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and _Nd values of 0 to +2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO₂ from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al₂O₃/TiO₂∼1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of _Nd values, from −2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. Received: 6 January 1998 / Accepted: 29 June 1998     

Geology and geochemistry of Pachmarhi dykes and sills,Satpura Gondwana Basin,central India: problems of dyke-sill-flow correlations in the Deccan Traps

Many tholeiitic dyke-sill intrusions of the Late Cretaceous Deccan Traps continental flood basalt province are exposed in the Satpura Gondwana Basin around Pachmarhi, central India. We present field, petrographic, major and trace element, and Sr–Nd–Pb isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically defined formations in the southwestern Deccan (Western Ghats). Some of these formations have also been identified more recently in the northern and northeastern Deccan. However, the Pachmarhi intrusions are significantly more evolved (lower Mg numbers and higher TiO₂ contents) than many Deccan basalts, with isotopic signatures generally different from those of the chemically similar lava formations, indicating that most are not feeders to previously characterized flows. They appear to be products of mixing between Deccan basalt magmas and partial melts of Precambrian Indian amphibolites, as proposed previously for several Deccan basalt lavas of the lower Western Ghats stratigraphy. Broad chemical and isotopic similarities of several Pachmarhi intrusions to the northern and northeastern Deccan lavas indicate petrogenetic relationships. Distances these lava flows would have had to cover, if they originated in the Pachmarhi area, range from 150 to 350 km. The Pachmarhi data enlarge the hitherto known chemical and isotopic range of the Deccan flood basalt magmas. This study highlights the problems and ambiguities in dyke-sill-flow correlations even with extensive geochemical fingerprinting.     

Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source,Mt. Erciyes Stratovolcano,Turkey

The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe₂O₃, MgO, CaO, low K₂O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low ⁸⁷Sr/⁸⁶Sr (0.703344-0.703964), and high ¹⁴³Nd/¹⁴⁴Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high ⁸⁷Sr/⁸⁶Sr (0.704591-0.70507) and low ¹⁴³Nd/¹⁴⁴Nd (0.51272-0.512394) isotopic ratios.

The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %.

The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.