Melting Of Asthenospheric Mantle Research Articles

SHRIMP zircon U-Pb ages of Kalatongke No. 1 and Huangshandong Cu-Ni-bearing mafic-ultramafic complexes, North Xinjiang, and geological implications

The SHRIMP zircon U-Pb dating was carried out and yielded 287±5 Ma (MSWD = 0.34) and 274±3 Ma (MSWD = 1.35) for the Kalatongke No. 1 and Huangshandong Cu-Ni-bearing mafic-ultramafic complexes. These ages are much more precise than pre-existing rock-mineral Rb-Sr, Sm-Nd and Re-Os isochron ages for the two complexes and constrain the timing of not only the complexes but also the magmatic Cu-Ni sulfide deposits more reliably. It is necessary to carefully reevaluate the previous chronological data for the complexes. The Cu-Ni-bearing mafic-ultramafic complexes have the ages similar to those of postcollisional A-type granites in the same area, implying that they could be related to the delamination of lithospheric mantle and upwelling and partial melting of asthenospheric mantle in postcollisional setting. Therefore, the Cu-Ni-bearing mafic-ultramafic complexes are a direct indicator of vertical growth of the continental crustal in the Central Asian Orogenic Belt.

Mantle plumes and flood basalts

We discuss the geological, geophysical, and petrological observations that constrain the nature of mantle convection in plumes, and show how theoretical models of mantle plumes have developed over the past three decades. The large volumes of lava emplaced in geologically short periods as flood basalts are generated mainly by decompression melting of abnormally hot mantle brought to the base of the lithosphere by plumes. We present new results from the application of McKenzieand O'Nions' (1991) rare earth element inversion scheme to the geochemistry of flood basalts to infer the amount of mantle melting and the depth interval over which it occurred. Our survey covers flood basalts from the Archaean to the Tertiary. The abundance of geochemical data from the Siberian Traps (250 Ma) and from the Keweenawan (1095 Ma) enables us to infer the temporal evolution of mantle melting in these two flood basalt provinces. We find that three quarters of the samples from flood basalts can be modeled by single‐stage melting of asthenospheric mantle. The remainder require enrichment by small amounts of metasomatic melts emplaced in the lithospheric mantle by an earlier phase of minor (0.3%) melting at depths where garnet is stable. The mantle melting responsible for flood basalts starts at depths of 110 km or more beneath the surface, and is consistent with enhanced mantle potential temperatures of 1450–1550°C. Melting continues to depths of 70–30 km. At least some lithospheric thinning is required to explain both the geochemistry of the melts and the high rate of generation of flood basalts.

Spatial migration and compositional changes of Miocene‐Quaternary magmatism in the Western Grand Canyon

Lithospheric extension appears to be responsible for erosional and tectonic evolution of the western margin of the Colorado Plateau. Late Cenozoic (20 Ma to 10,000 years) basaltic flows, scoria cones, necks, dikes, and sills exposed in the Western Grand Canyon region offer a unique opportunity to place age constraints on the landscape evolution. New K‐Ar ages show that these basaltic rocks become progressively younger to the northeast from the Hualapai and Shivwits Plateaus onto the Uinkaret Plateau, a distance of about 100 km. Magmatism appears to have migrated at a rate of about 0.45 cm/yr until sometime between 13.5 and 9.2 Ma, when the rate changed to about 1.15 cm/yr. Recent regional geologic mapping indicates that extensional faulting, with ubiquitous downdrop to the west, accompanied this magmatism; the Hurricane Fault offsets lavas as young as 0.29 Ma. Young fault scarps in sedimentary rocks, and historic seismic activity along these faults, indicate that the region is still undergoing faulting. The basalts form three relatively distinct groups based on their age, chemistry, and geographic distribution: (1) 20–11 Ma, (2) 11–4.5 Ma, and (3) ≤4.5 Ma. The older basalts (>11 Ma) are petrographically and compositionally distinct from younger basalts; their olivine and augite are more Mg rich with lower TiO2 and higher Ni, Cr, and incompatible element contents. Group 1 basalts have more radiogenic Sr and less radiogenic Nd isotopic compositions than younger group 2 and 3 basalts. Overall, present‐day Sr isotopic compositions range from 0.70344 to 0.70505, 143Nd/144Nd from 0.51291 to 0.51227 (εNd = +5.4 to −7.1), and 206Pb/204Pb from 17.951 to 19.091. Group 1 basalts were probably derived from asthenospheric mantle melts that underwent crustal contamination. These basalts appear to have been generated originally from more primitive melts than groups 2 and 3, which were probably also derived from asthenospheric mantle‐derived magmas but underwent only minor crustal contaminations. Group 2 basalts appear to be transitional between groups 1 and 3. The intervals 13–9 Ma and 4.6–4.3 Ma temporally separate the three chemically distinct groups of basalts in the Western Grand Canyon. These periods correspond closely with major Miocene and Pliocene plate reorganizations in the western United States at about 12.5–10 Ma and 3.9–3.4 Ma. These changes in plate motion were reflected in changes in the rate of magmatism migration in the Western Grand Canyon. Recent movements on the Hurricane and Toroweap Faults appear directly related to the <4.5 Ma volcanism and are probably a result of recent plate reorganization.

Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

Slab breakoff is the buoyancy‐driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low‐degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike‐slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high‐pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high‐pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

A Binary Source Model for Extension-Related Magmatism in the Great Basin, Western North America

Models for extension-related magmatism based on decompression melting of asthenospheric mantle poorly simulate fluxes and bulk compositions of magmas produced during early stages of continental extension. For the Great Basin of western North America, it is proposed that magmatism proceeded in two stages, the first involving melting of lithospheric mantle sources between 40 and approximately 5 million years ago (Ma), followed (since approximately 5 Ma) by melting of upwelling asthenospheric mantle in areas where extension has exceeded about 100 percent. This transition in magma sources is diachronous, depending on initial variations in lithosphere thickness and on rates of lithospheric thinning.

Geochemistry of mafic lavas in the early Rio Grande Rift, Yarmony Mountain, Colorado, U.S.A.

Mafic lavas (high-K basalts and shoshonites) erupted 21–24 Ma ago (early Miocene) near State Bridge, NW Colorado, and now exposed on Yarmony Mountain, were related to the initial phase of extension in the northernmost section of the Rio Grande Rift. The volcanism was of small volume, consisting of eleven flows on Yarmony Mountain, although much more voluminous contemporaneous lavas occur 50 km to the west. All eleven flows have been analysed comprehensively for major and trace elements, and five of the flows for Nd and Sr isotopes. Some samples experienced post-eruptive carbonation and leaching of alkali elements; the effects of these processes on normative compositions are examined, and it is suggested that major-element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid-lower part of the crust. Most of the basalts are chemically closely related, and have the following characteristics: (1) they are depleted in Nb and Ta relative to LREE, a feature of volcanic arc magmas; (2) they have lower abundances of Rb and K, and to a lesser extent Ba and Th, for a given LREE content, than typical magmatic arc magmas; and (3) they have Nd and Sr isotopic ratios similar to both certain low- 143 Nd 144 Nd oceanic basalts, notably from the Kerguelen area, Indian Ocean, and subduction-related magmas from mature volcanic arcs, notably from the Andes, South America. We argue that the magmas erupted at Yarmony Mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the Cenozoic. The relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction, before that which generated the Yarmony magmas. An alternative is that the mantle source was depleted in alkali elements by a dehydration event, possibly involving a flow of CO 2. One of the lavas in the Yarmony sequence is elementally and isotopically distinctive, having higher K, Zr, Ba, K 2 O Na 2 O , La/Ta and K/La, and lower 87 Sr 86 Sr than the rest of the samples. It is argued in detail that this is not the result of contamination by any reasonable crustal composition, and that this magma contained a component (∼10–20%) of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle. There is no petrographic evidence for mixing, and the hybridization probably took place in magma chambers situated in the mid-lower crust.