Peter R. Hooper

Stratigraphy, composition and form of the Deccan Basalts, Western Ghats, India

In the Western Ghats between latitudes 18° 20′ N and 19° 15′ N, 7000 km2 of Deccan Basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts. Using over 70 measured vertical sections, major and trace element analyses of nearly 1200 samples, and rare-earth and87Sr/86Sr determinations for over 60 samples, we divide the basalt into three subgroups and ten formations. In this paper we describe the seven principal formations in the area and the most prominent individual flows.The Kalsubai Subgroup is formed by the lower five formations, the Jawhar, Igatpuri, Neral, Thakurvadi, and Bhimashankar formations, from botton to top. In these formations amygdaloidal compound flows predominate and have a typically high MgO content, including picrite basalt (> 10% MgO) and picrite (> 18% MgO) with phenocrysts of olivine and clinopyroxene. These flows are separated by others which contain giant plagioclase phenocrysts and have more evolved chamical compositions.The Lonavala Subgroup overlies the Kalsubai and is composed of two formations, the Khandala and the Bushe. Both are readily recognized in the field and by their chemical compositions.The Wai Subgroup includes the upper three formations, the Poladpur, the Ambenali, and the Mahabaleshwar. The whole subgroup is composed of simple flows with well-developed flow tops, small phenocrysts of plagioclase, pyroxene and olivine, and relatively evolved bulk compositions.Distribution and variation in thickness of the straitigraphic units within the Western Ghats provide a first comprehensive view of the development of the Deccan volcanic edifice. The persistent southerly dip and gentle southerly plunging anticlinal form of the flows, the lensoid shape of many of the formations, and nearly randomly oriented feeder-dike system are together interpreted as evidence of a central volcanic edifice formed as the Indian plate drifted northward over a mantle plume or hot spot.

A role for lower continental crust in flood basalt genesis? Isotopic and incompatible element study of the lower six formations of the western Deccan Traps

Flows of the lower six formations of the western Deccan Traps (Jawhar through Khandala) cover a range of ϵNd(T) from 0 to −20, (87Sr86Sr)T 0.7062 to 0.7128 and 206Pb204Pb from 16.72 to 22.43. Oxygen isotopic data for fresh clinopyroxene and plagioclase separates indicate magmatic δ18O values between +6.6 and +7.4%.. Previous isotopic studies of the upper four formations (Bushe through Mahabaleshwar) have revealed two major trends that, to a first approximation, correspond to variable contamination of ϵNd(T) ≥ +7 source magmas by two very different negative ϵNd lithospheric endmembers. Isotopic data for the lower six formations describe completely different arrays from those of the upper formations and the fields for the individual lower formations are also distinct from one another. Significantly, the lower formation arrays overlap at or converge toward a common range of isotopic signatures, with ϵNd(T) ≈ 0.0 to −5.5, (87Sr86SrT ≈ 0.7067 to 0.7085, and 206Pb204Pb ≈ 19.2 to 20.9. These values are unlike those of oceanic mantle and intermediate between the extremes defined by the littlecontaminated Ambenali and highly crustally contaminated Bushe formations in the upper part of the stratigraphic sequence. One explanation for this common signature is that it represents a mantle source located in the continental lithosphere and quite distinct from the Ambenali-like source dominating the upper formations. However, the incompatible element patterns of the common-signature (and other lower formation) samples do not resemble those of typical Proterozoic or Phanerozoic continental mantle xenoliths and, unlike the Ambenali basalts, all of the lower formation samples analyzed to date have significantly higher δ18O values than oceanic lavas or the great majority of continental lithospheric mantle xenoliths. An alternative possibility is that the common signature magmas could be the products of a large-scale, open-system, lower crustal contamination process similar to that postulated for the thick mafic complex in the Ivrea Zone of northern Italy. If so, then the isotopic arrays emanating from the common signature would represent secondary contamination episodes involving at least three different crustal endmembers. In the upper formations, a two-stage mixing process also appears necessary to account for the Pb-Nd and Pb-Sr isotopic relationships displayed by data for the Bushe and Poladpur formations. Model calculations indicate that incompatible element patterns and isotopic ratios similar to those of the common-signature samples can be produced, while still maintaining a basaltic major and compatible trace element composition, by mixing a large-degree partial melt (~40%) of Indian Archean basic amphibolite into Ambenalitype or Reunion-type primitive magma. With the particular amphibolite composition used, the proportion of contamination required is large: roughly 10–30%, comparable to the amounts proposed for the mafic complex in the Ivrea Zone. More siliceous contaminants permit smaller amounts of contamination but generally yield poorer trace element fits.

Ages of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon

Stratigraphic and chemical correlations of Tertiary volcanic units in eastern Oregon confirm that the Steens Basalt represents the earliest eruptions of the Columbia River flood-basalt province. Field correlations are supported by major and trace element analyses and confirmed by 4 0 Ar/ 3 9 Ar dates. Within the basalt of Malheur Gorge, situated between Steens Mountain and the southernmost extent of the previously mapped Columbia River Basalt Group, the lowest unit correlates with the Steens Basalt, and the conformably overlying middle and upper units correlate with the Imnaha and Grande Ronde Basalt Formations of the Columbia River Basalt Group. New dates indicate that Imnaha and Grande Ronde Basalt Formations on the Columbia Plateau (>90% of the Columbia River Basalt Group) erupted between 16.1 and 15.0 Ma. These were immediately preceded by the Steens Basalt, a plagioclase-phyric tholeiite that erupted above the calculated position of the Yellowstone hotspot at 16.6 Ma. In eastern Oregon, the flood-basalt tholeiites of Steens Mountain and Malheur Gorge form a voluminous but brief interlude (16.6-15.3 Ma) superimposed on the low-volume, calc-alkalic to mildly alkalic, volcanism associated with continuing Eocene to present east-west extension.

The Columbia River Basalts

Between 17 million and 6 million years ago, 200,000 square kilometers of the American Northwest were flooded by basaltic lava that erupted through fissures in the crust up to 150 kilometers long. Larger individual eruptions covered over a third of the Columbia Plateau in a few days. The lavas represent partial melts of the earths mantle that were only slightly modified by near-surface, upper crustal processes. The abundant chemical and mineralogical data now available offer an opportunity to study mantle composition and the processes involved in the evolution of the earths crust.

The Madinah eruption, Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types

During a 52-day eruption in 1256 A.D., 0.5 km3 of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two.Three mineral assemblages occur out of equilibrium in all three chemical types.Assemblage 1 contains resorbed olivine and clinopyroxene megacrysts and ultramafic microxenoliths (Fo90 + Cr spinel + Cr endiopside) which fractionated within the spinel zone of the mantle.Assemblage 2 contains resorbed plagioclase megacrysts (An60) with olivine inclusions (Fo78) which fractionated in the crust.Assemblage 3 contains microphenocrysts of plagioclase and olivine in a groundmass of the same minerals with late-crystallizing titansalite and titanomagnetite; assemblage 3 crystallized at the surface and/or in the upper crust. Each assemblage represents a distinct range in PTX environment, suggesting that their coexistence in each chemical type may be a function of magma mixing. Such a process is confirmed by variable ratios of incompatible element pairs in a range of analyses.All three chemical types are products of mixing. Some of the hybrid types may have developed from surface mixing of the low-K and high-K lavas; however, the occurrence of all three types at the vent system suggests that subsurface mixing was the dominant process. We suggest that the Madinah flow was extruded from a heterogeneous magma chamber containing vertically stacked sections equivalent to the six eruptive pulses. This chamber may have developed contemporaneously with magma mixing when a crustal reservoir containing a magma in equilibrium with assemblage 2 was invaded by a more primitive magma containing cognate microxenoliths and megacrysts of assemblage 1.

Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.

The Columbia River Basalt

The Columbia River Basalt Group (CRBG) is one of the younger continental flood basalt provinces (17.5–6.0 My B.P.). The flows form a high plateau in northwestern USA, covering large parts of the states of Washington, Oregon, and Idaho between the actively rising Cascade Range to the west and the main ranges of the Rocky Mountain system to the east (Fig. 1). Of moderate size (164,000 km2 and an estimated 170,000 km3; Tolan et al., 1987) the CRBG is an order of magnitude smaller than the Karoo or Deccan provinces. The flows are fresh, almost horizontal, and well preserved by a semi-arid climate. Deep canyons, cut by rivers rising in the mountains to the east, provide a plexus of natural cross sections. These geographic advantages, combined with a systematic research effort by many groups over the last fifteen years, have provided a detailed picture of the physical development of the province and of its chemical and isotopic composition.

Composition of the Mount St. Helens Ashfall in the Moscow-Pullman Area on 18 May 1980

Mineralogical and chemical analyses of the ashfall from Mount St. Helens on 18 May 1980 indicate that there were two distinct ashes. The early dark ash is composed principally of plagioclase and lithic fragments of plagioclase and glass with titanium-rich magnetite and some basaltic hornblende and orthopyroxene. The later pale ash, four-fifths by weight of the whole fallout, is 80 percent glass with plagioclase as the principal crystalline phase. Quartz and potassium feldspar are rare to absent in both ashes. Chemical analyses of nine ash fractions and of the glass in each type emphasize the differences between the two ash types and their chemical homogeneity.

Physical and chemical constraints on the evolution of the Columbia River basalt

Physical and chemical constraints require that Columbia River basalt flows were fed from a large (>700 km 3 ) magma reservoir near the crust-mantle boundary and prohibit significant crystal fractionation or crustal assimilation between reservoir and surface. Demonstrable fractionation of plagioclase + olivine ± pyroxene in the reservoir largely obviates the need for an unusual iron-rich pyroxenite source. Variation of Sr and Nd isotope ratios with age in the main series of Columbia River basalt and variation in such incompatible trace-element ratios as Ba/P suggest lower crustal assimilation at the top of the reservoir combined with crystal fractionation. However, decoupling between isotope ratios and incompatible element ratios implies two separate processes in addition to crystal fractionation, of which lower crustal assimilation may be one. The other process may involve the participation of a vertically heterogeneous mantle resulting from metasomatism in the continental lithosphere.

Petrogenesis of the Colville Igneous Complex, northeast Washington: Implications for Eocene tectonics in the northern U.S. Cordillera

Eocene igneous complexes of the U.S. and Canadian Cordillera have long been thought to represent a broad and somewhat discontinuous subduction-related magmatic arc, extending from northern British Columbia to Wyoming. This model has become increasingly untenable as knowledge of the structural and tectonic settings of these rocks has expanded. New major, trace, and rare earth element and isotope data, presented here, virtually preclude the interpretation of a contemporaneous subduction-related source for mantle-derived magmas in the Colville Igneous Complex of northeast Washington State. We suggest an alternative model whereby post-Laramide orogenic collapse resulted in partial melting of a mid-crustal source, followed by partial melting of a distinctly different lower crustal source, the magma from which mixed with magma derived from a lithospheric mantle source. The subduction signature, and the calc-alkalic nature of the magmas, was inherited from previous Proterozoic subduction events.