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

The northeastern Deccan Traps include flows with strong isotopic and elemental affinities to lavas in the southwest as far as 900 km away. However, many of the northeastern flows show a negative correlation of 206Pb/204Pb with εNd(t) and positive correlation of 206Pb/204Pb with (87Sr/86Sr)t, distinct from any of the isotopic fields for the southwestern formations; this array (from εNd(t)=+5, (87Sr/86Sr)t=0.7041, 206Pb/204Pb=17.9 to εNd(t)=0, (87Sr/86Sr),=0.7068, 206Pb/204Pb=20.5) overlaps the Ambenali Formation (Fm.) field and runs toward that of the “common signature” [Peng et al., 1994] that appears to have been an important magma type in several of the lower formations of the southwest. This array appears to represent mixing between an Ambenali‐type and (1) a common‐signature end‐member or (2) an end‐member with even higher 206Pb/204Pb and 87Sr/86Sr and lower εNd than the common signature; such mixing has been postulated, but not documented previously, for the Deccan. Lavas isotopically and chemically indistinguishable from the Ambenali Fm. form the tops of sections near Jabalpur and Chikaldara. Beneath them are flows that chemically resemble the Poladpur Fm. (which sits immediately below the Ambenali in the southwest) and several lavas similar to the Khandala Fm. (beneath the Poladpur and intervening Bushe Fm. in the southwest). Given the general stratigraphic correspondence with the southwestern sequence, many of these northeastern flows may be petrogenetically related to the respective southwestern formations. Lavas with broadly Poladpurand Khandala‐like elemental compositions are also abundant in a thick section south of Mhow; several other Mhow‐area flows resemble the distinctive Bushe Fm. However, nearly all of the Poladpur‐ and Khandala‐like northeastern Deccan lavas analyzed have higher 206Pb/204Pb man any southwestern Poladpur or Khandala Fm. basalts. If they indeed are related petrogenetically to these formations, many of the northeastern magmas interacted less with lOw‐206Pb/204Pb continental material and thus must have erupted from different feeder systems.

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“…Star is for Rajmachi member and triangle is for Dhak Dongar member of southwestern Deccan [Peng et al, 1994;Peng, 1998]. …”

Section: Discussionmentioning

confidence: 99%

Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy

Peng¹,

Mahoney²,

Hooper³

et al. 1998

J. Geophys. Res.

116

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“…The Mahabaleshwar component is thought not to be ocean island mantle, because the lower limit of the Mahabaleshwar Fm. ( 206 Pb/ 204 Pb as low as ~16.7) is much lower than observed in modern oceanic lavas (Peng et al, 1994). Could the Ambenali magma type basically be N-MORB-type magma contaminated by small amounts of Nb-enriched (possibly phlogopite-bearing) continental lithospheric mantle, as Sheth (1999b) argued?…”

Section: Trace Element Modelling Of the Ambenali Magma Typementioning

confidence: 96%

“…1) in the Western Ghats (Sahayadri) range, and the stratigraphic framework of this region is now known quite well from extensive field, geochemical (including isotopic) and palaeomagnetic work (e.g., see Mahoney, 1988 and references therein;Subbarao, 1988 and references therein;Lightfoot et al, 1990;Peng et al, 1994). The Western Ghats sequence is almost exclusively tholeiitic (subalkalic) (Fig.…”

Section: The Deccan Tholeiites and Picritesmentioning

confidence: 99%

“…The field for Jabalpur lavas is from Peng et al (1998). The thick arrows marked 1 and 2 show the two stages of magma-lithosphere mixing conceived by Peng et al (1994Peng et al ( , 1998.…”

Section: Melting Of Eclogite-peridotite Mixturesmentioning

confidence: 99%

“…Significant crustal contamination has occurred in the petrogenetic evolution of many Jawhar to Poladpur Fm. basalts (e.g., Beane et al, 1986;Peng et al, 1994), and can be told from many criteria, one of them Sr isotopic ratios (Table 1). Therefore the positions of many samples below the ∆Nb = 0 line is not surprising.…”

Section: Identifying a Recycled Component In The Western Ghats Lavas:mentioning

confidence: 99%

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Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere?

Sheth¹

2005

Gondwana Research

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Deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust, and continental flood basalts (CFBs) are now thought by some to be derived from such eclogite-bearing peridotite plumes. Eclogite-peridotite mixtures have much lower solidi (and produce much greater melt fractions for a given temperature) than peridotite. Ferich (eclogite-or pyroxenite-bearing) sources have been inferred for many CFBs. However, plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite. Besides, CFBs are always located along pre-existing lithospheric structures (suture zones, edges of thick cratons) and commonly associated with lithospheric rifting and continental breakup. India's major late Mesozoic CFB, the Deccan Traps, erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent. Many Deccan basalts are too Fe-rich to have been in equilibrium with a peridotite mantle source, and have commonly been considered to be significantly fractionated derivatives of picritic liquids. However, it is possible to view them as relatively less evolved liquids derived from a source with extra fertility (i.e., an Fe-rich source). A new non-plume, plate tectonic model for Icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized Iapetus oceanic crust formerly trapped along the Caledonian suture. The model explains the geochemical-petrological characteristics of Icelandic basalts, and is consistent with passive upper mantle upwelling under Iceland inferred from recent seismic tomography. Based on the petrological and geochemical features of the Deccan flood basalts of the type section, in the Western Ghats, I propose that old, eclogitized oceanic crust trapped in the ancient Indian suture zones could have produced voluminous basaltic melts during the Deccan event.

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Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia

Lee

Chen

Zeng

2015

Geochem Geophys Geosyst

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Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K 2 O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of the lower crust (unradiogenic Pb, and low Ce/Pb), in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening nonconvecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution toward more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers.

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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

Cited by 227 publications

References 76 publications

Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy

Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy

Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere?

Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia

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