Bunbury Basalt Research Articles

The Kerguelen mantle plume activity in Sylhet Trap mafic rocks of Southern Shillong Plateau,NEIndia: Implications for its role in magmatism of eastern India

Mantle plumes related to Large Igneous Provinces have been linked to continental break‐up and validated by the outpouring of mafic‐ultramafic magmas that range from continental flood basalt magmatism to submarine plateau volcanism. This study presents a new set of geochemical and mineralogical data on mafic magmatic rocks from the Sylhet Trap of the Shillong Plateau, northeast India. The investigated mafic rocks (basalt and dolerite) are predominantly sub‐alkaline‐tholeiitic, composed of bytownite+labradorite and diopside+augite, with ophitic to sub‐ophitic and glomeroporphyritic textures, the dark interstitial region of much finer grains consisting of opaque minerals and devitrified glass. The mafic rocks of Sylhet Trap show light rare earth elements enrichment with (La/Yb)Nratio (1.92–2.86) and (La/Sm)Nratio (1.11–1.40), an almost flat pattern of heavy rare earth elements along with mild europium anomalies (Eu/Eu*= 0.94–1.11). Trace element characteristics suggest their affinity towards enriched mid‐oceanic ridge basalt and generated from low degree of partial melting of spinel source with minor involvement of crustal contamination. The similarity in geochemical characteristics of the investigated mafic rocks with the magmatism of Rajmahal Traps, eastern Peninsular India, Abor Volcanics, eastern Himalaya, along with Bunbury Basalt of western Australia and Cona Mafic exposed in southeastern Tibet, suggests their genetic linkage with mantle plume activities. Thus, we argue that the magmatic event of the Sylhet Trap is related to the Kerguelen mantle plume activity that played a significant role in the fragmentation of eastern Gondwana during the Lower Cretaceous period, giving rise to Greater India, Antarctica and northwest Australia.

Paleomagnetic constraints on the link between the Comei-Bunbury large igneous province and the Kerguelen mantle plume

The causal link between the Comei-Bunbury large igneous province (CBLIP) and the Kerguelen mantle plume (KMP) remains a topic of debate. Here, a new paleomagnetic study was conducted on the Zhela Formation basalts and sandstones (ca. 147–140 Ma) in the Taga area of the eastern Tethyan Himalaya (TH). The sandstone specimens had erratic demagnetization patterns, and reliable characteristic remanent magnetization (ChRM) directions could not be isolated. Positive fold and reversal tests indicate that the ChRM directions of the Zhela Formation basalts should be the primary magnetization. The tilt-corrected site-mean direction from 18 sites is D/I = 87.8°/70.0° (α95 = 6.8°), yielded a paleopole of 21.6°N/130.2°E (A95 = 12.0°) and a paleolatitude of 54.7° ± 12.0°S for the study area (28.4°N, 91.8°E). Comparison with robust latest Jurassic and Early Cretaceous paleomagnetic, geochemical, and geochronological data from the eastern TH, northeastern Indian craton, and southwestern Australia reveal that (1) the eruption periods of the Comei igneous rocks are similar to the Bunbury basalt (and the Naturaliste Plateau volcanic rocks); (2) the original erupted positions of the Comei-Bunbury (CB) igneous rocks were in agreement with the reconstructed KMP; and (3) the CBLIP originated from the KMP.

Greater Kerguelen large igneous province reveals no role for Kerguelen mantle plume in the continental breakup of eastern Gondwana

The link between mantle plumes and continental breakup remain a topic of debate. Here, a new 40Ar/39Ar age of 135.9 ± 1.2 Ma (2σ) and previous ages from the Bunbury Basalt – lava flows that are part of the Greater Kerguelen large igneous province (LIP) – reveal that >80% of magmatism in the southern Perth Basin was concomitant with the continental breakup of eastern Gondwana at ca. 137–136 Ma. New and existing isotope geochemical data show that only lithospheric and depleted asthenospheric sources were melted to form the Bunbury Basalt and most other early, ca. 147–124 Ma magmatic products part of the Greater Kerguelen LIP. All lines of evidence strongly point towards passive continental breakup of eastern Gondwana, including the restriction of 147–124 Ma magmatism to continental rifts, the lack of excess oceanic magmatism in this period and the >1000 km distance between the Kerguelen plume underneath Greater Indian lithosphere and the breakup nexus. It is not possible to reconcile the influence of a thermochemical plume with the observed geochemical, spatial and geochronological information. Instead, we posit that eastern Gondwana breakup occurred because it was proximal to the former suture zone associated with the ca. 550–500 Ma Kuunga Orogeny between Indo–Australia and Australo–Antarctica. Enrichment of the mantle with volatiles associated with subduction during the Kuunga Orogeny permitted partial melting when the continental crust was sufficiently attenuated in the Early Cretaceous. Repeated and protracted rifting of Greater India from Australo–Antarctic since the mid-Paleozoic eventually led to the rupture of the continental lithosphere and to mafic magmatism at ca. 137–136 Ma, approximately along the position of the former suture zone, without the influence of a mantle plume.

Petrogenesis of the Early Cretaceous Kada igneous rocks from Tethyan Himalaya: Implications for initial break‐up of eastern Gondwana

Early Cretaceous magmatic rocks associated with the Kerguelen mantle plume are widespread in the Comei area with a chronology concentrated from 134 to 130 Ma. However, less data for the earliest Early Cretaceous magmatic rocks cropping out in the Comei area have been reported. The earliest Early Cretaceous magmatic rocks from the Kada area comprise mafic rock (diabase) and felsic rocks (granite, dacite, and quartz monzonite). This paper presents a detailed LA‐ICP‐MS zircon U–Pb chronology with major and minor (trace) elements and Lu–Hf isotope data from Kada igneous rocks to identify the extent of the early activity of the Kerguelen mantle plume and the initial break‐up of eastern Gondwanaland. The LA‐ICP‐MS zircon U–Pb dates reveal that the age of Kada magmatic rocks range from 136.9 to 143.5 Ma. Kada mafic rocks exhibit both OIB (Samples KD3 and KD06) and N‐MORB (Sample KD7‐1) characteristics. The N‐MORB‐type mafic rocks were derived from partial melting of depleted spinel lherzolite. However, the OIB‐type mafic rocks were derived from partial melting (~60–80 km) of spinel + garnet lherzolite. Kada felsic rocks show obvious enrichment of light rare earth elements and negative Eu anomaly features. Among these, dacite and quartz monzonite rocks show flat heavy rare earth elements and clear Nb, Ta, P, and Ti negative anomalies with depleted εHf(t) characteristics (−11.0 to −21.0). This is indicative of partial melting of specific crustal rocks in an intraplate environment caused by underplating of coeval basaltic magmas. However, granite is characterized by high heavy rare earth element fractionation and high and dispersed εHf composition (−1.5 to +8.9), thus indicating that granite is derived from extensive fractional crystallization of mantle‐derived melts from an AFC process. The Kada felsic rocks show A‐type granitic characteristics in an extensional environment. Based on the evidence from palaeomagnetism reconstructions, Kada rocks and Bunbury basalts are both distributed in and influence the scope of the Kerguelen mantle plume. They are also coeval with the initial break‐up of eastern Gondwana. We suggest that Kada magmatic rocks in southern Tibet represent the earliest activity of the Kerguelen mantle plume and that they played a role in the initial break‐up of eastern Gondwana.

Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

ABSTRACTVolcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ∼70 000 km2 Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ∼90 000 km2 Naturaliste Plateau, located ∼170 to ∼500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. 40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km2.

Bunbury Basalt: Gondwana breakup products or earliest vestiges of the Kerguelen mantle plume?

In this contribution, we investigate the role of a mantle plume in the genesis of the Bunbury Basalt using high-precision 40Ar/39Ar geochronology and whole-rock geochemistry, and by using crustal basement thickness of the eastern Indian Ocean and the western Australian continent. The Bunbury Basalt is a series of lava flows and deep intrusive rocks in southwestern Australia thought to be the earliest igneous products from the proto-Kerguelen mantle plume. Nine new plateau ages indicate that the Bunbury Basalt erupted in three distinct phases, at 136.96±0.43 Ma, 132.71±0.43 Ma and 130.45±0.82 Ma. All Bunbury Basalt samples are enriched tholeiitic basalts with varying contributions from the continental lithosphere that are similar to other Kerguelen plume-products. Based on plate reconstructions and the present geochronological constraints, the eruption of the oldest Bunbury Basalt preceded the emplacement of the Kerguelen large igneous province by at least 10–20 m.y. Such age differences between a precursor and the main magmatic event are not uncommon but do require additional explanation. Low crustal stretching factors beneath the Bunbury Basalt (β≈1.4) indicate that decompression melting could not have been generated from asthenospheric mantle with a normal chemistry and geotherm. An elevated geotherm from the mantle plume coupled with the geochemical similarity between the Bunbury Basalt and other Kerguelen plume-products suggests a shared origin exists. However, new age constraints of the oldest Bunbury Basalt are synchronous with the breakup of eastern Gondwana and the initial opening of the Indian Ocean at ca. 137–136 Ma, which may mean an alternative explanation is possible. The enriched geochemistry can equally be explained by a patch of shallow mantle beneath the southern Perth Basin. The patch may have been enriched during Gondwana suturing at ca. 550–500 Ma, during early rifting events by magmatic underplating or by intruded melts into the subcontinental lithospheric mantle. This enriched geochemical signature would then be sufficient to trigger decompression melting from passive rifting between Greater India and Australia with no contribution from the Kerguelen hotspot. We conclude that although the proto-Kerguelen hotspot is certainly a possible explanation for the genesis of the Bunbury Basalt, decompression melting of an enriched patch of subcontinental lithospheric mantle is an alternative theory.

Paleo-drainage and structural deformation during Gondwana breakup: insights from the 3D Geometry of the Bunbury Basalt

SUMMARY The Bunbury Basalt is a series of lava flows in the Perth Basin, Western Australia, and is interpreted to be related to the breakup of eastern Gondwana during the Early Cretaceous. We integrate new aeromagnetic images with available well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that the flows can be up to 100 m thick and are predominantly confined to two paleochannels and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook paleochannel flows proximal to the Darling Fault and is cut by the Bunbury paleochannel, which is positioned centrally in the Bunbury Trough and contains two flow episodes. The model illustrates a dominantly north–south axial paleo-drainage pattern that existed in the southern Perth Basin prior to Gondwana breakup. The 3D model provides a presentday volume of 90 km 3 with an aerial extent of 3300 km 2 . Offsets of the Bunbury Basalt have been used to identify new northeast and northwest trending faults in the southern Perth Basin, and broad folding is interpreted to a consequence of drag into the Darling and Busselton Faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling and Busselton Faults with subordinate oblique faults.

Petrogenesis of the Early Cretaceous Laguila bimodal intrusive rocks from the Tethyan Himalaya: Implications for the break-up of Eastern Gondwana

The Kerguelen mantle plume triggered the rift of Eastern Gondwana to open the eastern Indian Ocean, with the formation of ~132Ma Comei–Bunbury large igneous province (LIP). The Comei area is located in the eastern Tethyan Himalaya, paleogeographically belonging to Greater India. The Laguila bimodal intrusive rocks from the Comei area consist of mafic (gabbro-diabase) and felsic rocks (quartz monzonite-granodiorite). This paper presents detailed LA-ICP-MS zircon U-Pb chronology, major and trace elements, and Sr-Nd-Pb isotope geochemistry of the Laguila bimodal intrusive rocks, in order to constrain the early activity of the Kerguelen mantle plume. LA-ICP-MS zircon U-Pb dating shows that the Laguila intrusive rocks were emplaced in the Early Cretaceous (~134–130Ma). The Laguila mafic rocks are enriched in LREE, LILE and HFSE, similar to those of oceanic island basalts (OIB). Their 87Sr/86Sri (0.7054 to 0.7066), 143Νd/144Nd (T) (0.512548 to 0.512619) and (206Pb/204Pb)t ratios (18.492 to 18.859) are comparable with those basalts derived by the Kerguelen hot spot. Elemental and isotopic data suggest that they were likely derived by partial melting of the Kerguelen plume source in the spinel–garnet transition zone (~60–80km). The Laguila felsic rocks share most of the geochemical features of A-type granite and show different 87Sr/86Sri (0.7171 to 0.7204), 143Νd/144Nd (T) (0.511874 to 0.511956) and (206Pb/204Pb)t ratios (19.087 to 19.274) from those of the mafic rocks. They were likely derived by partial melting of crustal rocks at a shallow depth (<30km) triggered by underplating of the coeval basaltic magmas. The Laguila intrusive rocks were emplaced in a rift setting during the breakup of eastern Gondwana, associated with the Kerguelen plume activity.We calculated the magmatic volume of Comei–Bunbury basalts and the result is ~1.1×104km3. The small volume is not reconciled with those typical models for the initial magmatic eruption of mantle plume. It was possible that the thick lithosphere (>150km) underneath Greater India prevented the Kerguelen plume from large-scale melting because of the high pressure when the latter impinged onto the former at ~132Ma. The shallow source (~60–80km) of the Laguila mafic rocks indicate that they originated likely from a series of “diapirs” or “fingers” rising from the head of the Kerguelen plume, which penetrated into the thick lithospheric mantle of Eastern Gondwana at enough shallow depth for melting.

Paleodrainage and fault development in the southern Perth Basin, Western Australia during and after the breakup of Gondwana from 3D modelling of the Bunbury Basalt

The evolution of faults and paleodrainage patterns on the southwestern Australian passive margin during and after the breakup of Gondwana in the Early Cretaceous remains poorly understood. This contribution investigates the fault and paleodrainage evolution in the southern Perth Basin with the use of the ‘Bunbury Basalt’, the only lava flows known to be synchronous with continental breakup. New aeromagnetic data have been integrated with well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that flows are up to 100 m thick and are predominantly confined to two north–south-trending paleovalleys and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook Paleovalley flow ponded in a paleovalley proximal to the Darling Fault and is truncated by the two later flows within the Bunbury Paleovalley, which is positioned centrally in the Bunbury Trough. Offsets of the Bunbury Basalt have been used to identify new northeast- and northwest-trending faults in the southern Perth Basin, and broad folding is interpreted as a consequence of drag into the Darling and Busselton faults. The model has been used to determine post-basalt net displacements for the Darling and Busselton faults of 370 and 210 m, respectively, and <175 m for the northeast and northwest-trending faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling Fault and subordinate oblique faults. These results challenge the views on longstanding quiescence of the post-breakup western Australian passive margin.

Isotopic and trace element geochemistry of alkalic–mafic–ultramafic–carbonatitic complexes and flood basalts in NE India: Origin in a heterogeneous Kerguelen plume

The Archean East Indian cratonic margin was affected by the Kerguelen plume (KP) ∼117Ma, causing flood-basalt eruptions of the Rajmahal–Bengal–Sylhet Traps (RBST). The RBST cover ∼one million km2 in and around the Bengal Basin as alkalic–ultrabasic intrusives in the west and Sikkim in the north, and Sylhet basalts and alkalic–carbonatitic–ultramafic complexes in the Shillong plateau – Mikir hills farther east of the Rajmahal–Bengal Traps. We provide new Nd–Sr–Pb-isotopic and trace element data on 21 unreported discrete lava flows of the Rajmahal Traps, 56 alkalic–carbonatitic–mafic–ultramafic rocks from four alkalic complexes, and three dikes from the Gondwana Bokaro coalfields, all belonging to the RBST. The data allow geochemical correlation of the RBST with some contemporaneous Kerguelen Plateau basalts and KP-related volcanics in the southern Indian Ocean. Specifically, the new data show similarity with previous data of Rajmahal group I–II basalts, Sylhet Traps, Bunbury basalts, and lavas from the southern Kerguelen Plateau, indicating a relatively primitive KP source, estimated as: εNd(I)=+2, 87Sr/86Sr(I)=0.7046, with a nearly flat time-integrated rare earth element (REE) pattern. We model the origin of the uncontaminated RBST basalts by ∼18% batch melting with a 2× chondritic KP source in the spinel-peridotite stability depths of 60–70km in the mantle. The new geochemical data similar to the Rajmahal group II basalts indicate a light REE enriched average source at εNd(I)=−5, 87Sr/86Sr(I)=0.7069. Our geochemical modeling indicates these lavas assimilated granulites of the Eastern Ghats, reducing the thickness of the continental Indian lithosphere. Lack of an asthenospheric MORB component in the RBST province is indicated by various trace element ratios as well as the Nd-Sr isotopic ratios.Three alkalic complexes, Sung, Samchampi, and Barpung in NE India, and one in Sikkim to the north are of two groups: carbonatites, pyroxenites, lamproites, nephelinites, sovites, melteigite in the first group and syenites and ijolites in the second. The Nd–Sr–Pb-isotopic and trace element geochemistry of the first group of carbonatitic–ultrabasic rocks are consistent with similar data of the RBST lavas of the present and previous studies, and are modeled as derived from a relatively primitive carbonated garnet peridotite source in the KP. In contrast, the syenites and ijolites of the second group show a wide range of Nd–Sr–Pb isotopic compositions, modeled by low-degree melts of an ancient recycled carbonated eclogite also in the KP. The KP thus reflects heterogeneities in the lower mantle-derived plume with carbonated components yielding ultrabasic melts at greater depths with low-degree melting, followed by rise of the plume at shallower depths causing tholeiitic flood basalt volcanism. Collectively, these data imply a zone of influence of the plate-motion-reconstructed KP head for ∼1000km around the Bengal Basin, as represented by the widely scattered and diverse rock types of the RBST.

Vestiges of the Kerguelen plume in the Sylhet Traps, northeastern India

The 117 Ma Sylhet Traps, exposed on the southern edge of the Shillong Plateau in northeastern India, are separated from the Rajmahal Traps ~ 550 km to the west by the Gangetic-Brahmaputra alluvium of the Bengal basin. On the basis of their similar age, Sylhet and Rajmahal Traps are correlated. We report Nd–Sr–Pb-isotopic and multiple trace element data for 18 discrete and consecutive lava flows from two sections of the Sylhet Traps. Thirteen of the analyzed lavas are from the Cherrapunji-Shella (CH) Bazaar section and five from the Mawsynram-Balot (MB) section. In major, trace elements and Nd–Sr–Pb isotopes, most of these lavas show similarity with Rajmahal Traps, Bunbury basalts and lavas from Naturaliste and parts of the Kerguelen Plateaus, allowing reconstruction of a ~ 800 km Kerguelen plume-head in the Bengal basin aligned with the Ninetyeast Ridge. The combined geochemical data and their correlation with the Rajmahal Traps, Bunbury basalts, and some Kerguelen Plateau lavas, imply a relatively less depleted plume source for CH basalts. We assess the average composition of this source at 117 Ma to be: ε Nd(I) = 2, 87Sr/ 86Sr(I) = 0.7046, with relatively flat rare earth element patterns, similar to the basalts from the Ocean Drilling Sites 1138, 1141 and 1142 on the Kerguelen Plateau. The Nd–Sr-isotopic data for the Sylhet basalts are modeled with two end members, an 18% partial melt from a chondritic garnet peridotite source, and a granulitic contaminant of the Eastern Ghats Belt. Most of the Sylhet lavas are close to the proposed plume end-member. The contaminated Sylhet basalts reflect as much as 20% of the granulite component caused by the incorporation of lower-continental crust in the Kerguelen plume-derived melt. Combined Nd–Sr–Pb-isotopic evidence, and, in particular, Ce/Pb vs. ε Nd(I) mixing-models among different reservoirs indicate more primitive CH lavas to be mixture of bulk-chondritic Earth and E-MORB, without apparent mixing with N-MORB, continental crust, or non-chondritic bulk Earth. However, Sr–Pb-isotopic ratios of these lavas fall in the estimated ranges of non-chondritic bulk Earth. The least contaminated Kerguelen plume component may be common to other large igneous provinces.

The 132 Ma Comei-Bunbury large igneous province: Remnants identified in present-day southeastern Tibet and southwestern Australia

We report 11 new U-Pb zircon ages obtained by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP–MS) for a large province of Early Cretaceous Comei igneous rocks consisting of basaltic lavas, mafi c sills and dikes, and gabbroic intrusions together with subordinate layered ultramafi c intrusions and silicic volcanic rocks exposed in the Tethyan Himalaya, southeastern Tibet. Available zircon U-Pb ages obtained from various rocks in this province, which has an areal extent of ~40,000 km 2 (~270 km × 150 km), indicate that the magmatism occurred ca. 132 Ma ago, coeval with the Bunbury Basalt in southwestern Australia. Such a striking similarity in emplacement age, in combination with the tectonic reconstruction of eastern Gondwana ca. 132 Ma ago, allows us to propose that the extensive Comei igneous rocks in southeastern Tibet and the Bunbury Basalts in southwestern Australia may represent the erosional and/or deformational remnants of a large igneous province, which we call the Comei-Bunbury LIP. We argue that this newly identifi ed LIP was likely caused by the Kerguelen mantle plume, which started in the Early Cretaceous and may have played a role in the breakup of eastern Gondwana and the development of the 132 Ma old Weissert oceanic anoxic event.

SHRIMP U-Pb zircon dating for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya and its implications

Petrology and SHRIMP U-Pb zircon chronology are reported for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya to the southeast of Yangzuoyong Co. Twenty-one measured zircon grains from a dacite sample of the Sangxiu Formation can be divided into two groups, which include long columnar magmatic zircons of 133±3.0 Ma, representing the age of volcanism in the Sangxiu Formation, and inherited zircons consisting of core and overgrowth rim, in which ages of the three cores are 2244±16, 1153±33 and 492±25 Ma respectively; and the age of one overgrowth rim is 132.7±5.2 Ma. These ages are considered to represent the remelting of accretionary crust at different periods resulting from the volcanism of Sangxiu Formation. These volcanic rocks in the Sangxiu Formation, for which age is close to that of Bunbury basalt in southwestern margin of Australia, and older than that of Rajmahal basalt in northeastern India, maybe considered as the products of early activity of the hotspot represented by the Rajmahal basalt.

Origin of Cretaceous continental tholeiites in southwestern Australia and eastern India: insights from Hf and Os isotopes

Rifting between eastern India and western Australia in the Early Cretaceous (∼140–133 Ma) resulted in the formation of the eastern Indian Ocean. Tholeiitic flood basalts erupted on the southwestern margin of Australia (Bunbury basalts at ∼132 and ∼123 Ma) and the eastern margin of India (Rajmahal Traps at ∼118 Ma) shortly after this time, while the Kerguelen Plateau began to form in the nascent ocean basin (∼119 Ma). We studied the Hf and Os isotopic systematics of the continental basalts in order to better understand the relationship between these volcanic events and the Kerguelen plume. The objective of this work was to determine the mantle sources of the continental tholeiites and the nature and quantity of continental contamination. Both the Hf and Os isotopic compositions of these basalts vary with indicators of continental contamination (SiO 2, (Th/Ta) PM, (La/Nb) PM). There is no obvious correlation of γ Os( T) with Sr or Nd isotopic ratios, but rough correlations are present between Os and Hf isotopes and 208Pb/ 204Pb; in contrast, ε Hf( T) shows strong correlations with all isotopic systems and indicators of continental crust assimilation. We demonstrate that the mantle sources of these continental tholeiites cannot originate purely from the depleted asthenosphere. An enriched mantle source is required for both of these basalt suites and the geochemical characteristics of this source are broadly consistent with those inferred to be present in the Kerguelen plume during Early Cretaceous time. The Os isotopic results suggest that at least some continental contamination is present in all samples from both the Bunbury basalts and the Rajmahal Traps. In the Rajmahal Traps, this contaminant appears to be derived from the continental crust and there is little to no evidence for involvement of subcontinental lithospheric mantle (SCLM). The Bunbury basalts have consistently unradiogenic Os isotopic compositions and high ε Hf( T) for a given ε Nd( T), characteristics ascribed to the SCLM. Previous investigations of these basalt suites have concentrated on major and trace elements and Sr, Nd and Pb isotopes; none of these results suggested the involvement of SCLM in the Bunbury basalts. Here, we emphasize that the study of Hf and Os isotopes provide additional, useful tools for deciphering mantle sources and continental contaminants involved in the petrogenesis of continental tholeiites.

Geodynamic implications of moving Indian Ocean hotspots

The reconstructed positions of many of the Indian Ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed. The discrepancy can be explained by the past motion of the hotspots themselves, and in the errors introduced in an absolute plate model based on fixed hotspots. Previous models for the motion of hotspots in the Indian Ocean (e.g. [Steinberger and O’Connell, in: The History and Dynamics of Global Plate Motions, 2000, pp. 377–398, and Geophys. J. Int. 132 (1998) 412–434; Antretter et al., Earth Planet. Sci. Lett. 203 (2002) 635–650]) have had success in matching paleolatitudes of the Kerguelen hotspot, but not the Marion or Reunion hotspots. We calculate the motion of the Indian Ocean hotspots using a method described in Steinberger and O’Connell (op. cit.), employing a plate motion model revised using identification of Mesozoic magnetic anomalies in the Enderby Basin [Gaina et al., in: The Evolution and Dynamics of the Australian Plate, in press]. We find a motion of ∼7° south for the Kerguelen plume, similar to previous models [Antretter et al., op. cit.]. This motion alleviates the discrepancies in the position of the Kerguelen plume and its purported involvement in the formation of the Ninetyeast Ridge, Rajmahal Traps, Bunbury basalts and assorted volcanics of the Western Australian margin. We find a motion of 5° north and 7° south for the Reunion and Marion plumes, respectively. The motion of the Reunion hotspot is consistent with paleolatitude estimates. The apparent fixity of the Marion hotspot [Torsvik et al., Earth Planet. Sci. Lett. 164 (1998) 221–232] is not well-constrained, but may be due to the combination of hotspot motion and true polar wander acting in opposing directions. We present revised absolute finite rotations for the Indian plate for the last 65 Ma based on the motion of these hotspots, which place India farther north in the past than for fixed hotspot models.

Kerguelen Hotspot Magma Output since 130 Ma

The Kerguelen hotspot (Indian Ocean) has produced basalt for …130 Myr, among the longest known volcanic records from a single source. New and published 40Ar/39Ar age determinations from the Kerguelen Plateau, Broken Ridge, Rajmahal Traps, and Bunbury basalts, and of Indian and Antarctic dikes help to document the hotspot’s history. Using radiometric dates and crustal structure determined from geophysical data and drilling results, we calculate the magmatic output of the Kerguelen hotspot through time. Output rates have varied in ways not predicted by current geodynamic models; maximum eruption volumes postdate the initial surface manifestation of the hotspot as well as break-up between Antarctica and India by ∶15 Myr, and magma output rates were high, as well as geographically diverse, over an interval of 25 Myr, from …120 to …95 Ma. We propose two alternatives to the standard mantle plume paradigm, one involving multiple plume sources, and another consisting of a single, but dismembered plume source.

Relationship between the early Kerguelen plume and continental flood basalts of the paleo-Eastern Gondwanan margins

Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr–Nd–Pb isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from 5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments of continental crust, isolated at shallow levels in the Indian Ocean lithosphere.

Search for a deep-mantle component in mafic lavas using a Nb–Y–Zr plot

Concentrations of trace elements with bulk distribution coefficients &lt; 1 have been utilized to discriminate between mafic magmas erupted in different tectonic settings. Herein, the Nb–Y–Zr contents of mafic lavas will be utilized to pinpoint derivation from a deep-mantle source. Magmas derived from the Icelandic plume exhibit ΔNb &gt; 0, where ΔNb = log (Nb/Y) + 1.74 – 1.92 log (Zr/Y) (Fitton et al. 1997), whereas melts derived from depleted sections of the mantle and the crust show ΔNb &lt; 0. Regardless of the nature of the melting event, it was suggested the ΔNb value of magmas were essentially identical to that of the source material. Theoretical modeling of a garnet lherzolite source suggests very small partial melts may show elevated ΔNb values, yielding a spurious plumelike signature. For mafic rocks from different areas, with hypothesized hot spot derivation, ΔNb is calculated to search for a deep-mantle signature (ΔNb &gt; 0). The signature of the Réunion hot spot, from ~65–0 Ma, is seen in the Deccan Traps, India, lavas from the Mascarene Plateau, and Réunion Island. The signature of the Kerguélen plume is detected in rocks from Kerguélen Island and the Ninetyeast and Broken Ridges, Indian Ocean, a few lavas from the Rajmahal province, India, but none in the Bunbury Basalt, Australia. Some rocks from continental flood basalt provinces show a hot spot component, in agreement with conclusions based on 3He/4He studies, others show ΔNb &lt; 0, due to considerable subcontinental contamination. For lavas from Heard Island and Grand Comore, Indian Ocean, the technique pinpoints those showing a plume component. In an area of plate convergence in the Pacific Ocean, lavas with very low (&lt;1 ppm) Nb contents, serve as a critical test for this method. Only rocks close to the Samoan hot spot show ΔNb &gt; 0, whereas the others show the Nb depletion typical of arc-related material. In conjunction with trace-element and isotopic discriminants, this appears to be a useful tool in identifying a deep-mantle component in mafic lavas from a variety of plate tectonic settings.

Evolution of the Western Margin of Australia during the Rodinian and Gondwanan Supercontinent Cycles

The proto-Darling Fault zone and its successor, the Darling Fault, extend for 1, 000 km along the western continental margin of Australia and appear to have been active at several periods during the geological past. Deformation commenced at ∼2,570 Ma and affected Late Archaean granitoids along the western margin of the Yilgarn Craton. Much of the later activity reflects events related to the accretion and breakup associated with the Rodinia and Gondwanaland supercontinent cycles. In the north, rocks of the Northampton and Mullingarra Complexes form part of a high-grade Grenvillian orogenic belt lying to the west of the Darling Fault, referred to as the Pinjarra Orogen. They underwent granulite facies metamorphism ∼1080 Ma ago and form part of the global collisional event that resulted in the amalgamation of Rodinia. These rocks extend southward beneath Phanerozoic sedimentary cover (the Perth Basin), where they are constrained to the east by the Darling Fault and to the west by the Dunsborough Fault, the latter marking the eastern boundary of the Leeuwin Complex. The Leeuwin Complex is a fragment of Pan-African crust that has traditionally been considered part of the Pinjarra Orogen. It is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from A-type, anorogenic granitoids. Conventional and SHRIMP U-Pb zircon geochronology has established that the granitoids evolved between ∼780 Ma and ∼520 Ma and were metamorphosed at ∼615 Ma. These events are equated with rifting associated with the breakup of Rodinia. Sm-Nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre-existing Archaean continental material. During the Phanerozoic, the Dunsborough and Darling Faults were reactivated, as normal faults defining the inner arm of a major rift system within Eastern Gondwanaland and controlling sedimentation in the Perth Basin that now overlies the Grenvillian terrane. Major normal movement on the Darling Fault ceased by the Late Jurassic and it appears that continental breakup in the Early Cretaceous occurred along fractures closely related to the western boundary of the Leeuwin Complex that defined the eastern margin of the outer arm of the rift system. Breakup between Australia and Greater India commenced at ∼132 Ma and was followed by eruption of the Bunbury Basalt at 130 Ma and 123 Ma. This possibly resulted from hot spot activity beneath Eastern Gondwanaland and may have been a reflection of the Kerguelen plume, though the evidence is equivocal. It is argued from the petrographic, geochemical and isotopic characteristics, together with the likely contiguity of the Eastern Gondwanaland continents since the assembly of Rodinia, that the Leeuwin Complex evolved within an intracrustal rift and is not an exotic terrane. It is distinct from adjacent portions of the Pinjarra Orogen and should be considered a separate terrane. It is recommended that use of the term ‘Pinjarra Orogen’ be confined to rocks recording the Grenvillian events, thereby excluding those rocks (the Leeuwin Complex) that evolved during the later Pan-African orogeny.

Petrogenesis of the Bunbury Basalt, Western Australia: interaction between the Kerguelen plume and Gondwana lithosphere?

Subsequent to the initial rifting of Greater India from Australia/Antarctica at ∼ 132 Ma, widespread early Cretaceous volcanism occured on the continental margins. Two examples are the ∼ 117 Ma Rajmahal Traps in northeast India and the ∼ 123–130 Ma Bunbury Basalt in Western Australia, which erupted in the Perth Basin, a fault-bounded trough separating Archaean and Proterozoic crust. On the basis of spatial distribution, age, petrography, and geochemical characteristics the Bunbury Basalt is divided into two lava types: Casuarina and Gosselin. Relative to the 130 Ma Casuarina lavas, the 123 Ma Gosselin lavas have higher MgO content, lower abundances of high field strength elements (P, Ti, Zr, Nb and Ta), higher 87Sr/ 86Sr (∼ 0.7080), and 207Pb/ 204Pb (> 15.61) and lower 143Nd/ 14Nd . These isotopic features, combined with relative depletion in Nb and Ta, are characteristic of continental crust. The low MgO and Ni contents (5.3–6.9% and < 100 ppm, respectively) of all Bunbury lavas indicate that they experienced considerable crustal processing. Although the isotopically extreme Gosselin lavas have higher MgO contents than Casuarina lavas, their compositions reflect fractional crystallization at lower pressures than Casuarina lavas; we infer that the Gosselin magmas evolved by fractional crystallization and crustal assimilation within the upper crust. The present-day Sr and Nd isotopic ratios in Casuarina lavas are similar to those in younger lavas from the Ninetyeast Ridge and Kerguelen Archipelago which define the Kerguelen hotspot track. These isotopic similarities are consistent with a genetic relationship to the Kerguelen plume, but relatively high 207Pb/ 204Pb ratios (15.57 – 15.62) indicate that Casuarina lavas contain a small amount of the crustal component present in Gosselin lavas. Although it is possible that the Bunbury Basalt is a response to long-term incubation of the plume beneath eastern Gondwana, the eruption ages of Bunbury lavas (123 and 130 Ma) significantly exceed the oldest measured ages (∼ 110–115 Ma) for the large igneous province (Kerguelen Plateau) associated with the plume; moreover, Western Australia was apparently ∼ 1000 km from the plume during this time. Therefore, the Bunbury Basalt may be unrelated to the plume; in this case, the geochemical similarities of Casuarina and younger plume-related lavas are fortuitous.