Mantle Contamination Research Articles

The emplacement, geochemistry and petrogenesis of two central Morocco Hercynian granites. Geotectonic implications

New field, petrographic and geochemical data including REE, Rb-Sr, and K-Ar, are presented concerning the emplacement and petrogenesis of two calc-alkaline Moroccan Hercynian granites: the Zaër pluton and the Djebel Aouam stocks. Zonation in the Zaër pluton does not appear to result from simple fractional crystallization nor from hydrothermal alteration but is rather the diapiric intrusion of two interlocked bodies. REE geochemistry supports the interpretation that the biotite-granodiorite magma (301 ± 8.2 M.a.) and the Djebel Aouam stocks appear to have been derived by fusion of crustal materials with possible mantle contamination ( Initial 87 Sr/ 86 Sr ratio = 0.70514 ). The second two-mica granitic magma (283.4 ± 6.2 M.a.) corresponds to a peraluminous granite (1.22 < A/ CNK < 1.33) generated by fusion of continental crust including mature sedimentarr materials (Initial 87Sr/ 86Sr ratio = 0.70836). These granites are compared to the Acadian granites of north Appalachian and classified in the «post-collision, uplift environmentå type. An illustrative four step model presenting the generation and emplacement of these two granites together with the associated W-Sn-Pb-Zn-Ag mineralizations is proposed: (1) first magma generation, (2) first melt emplacement, (3) second melt emplacement, (4) late convective flow.

Light rare-earth enrichment of supra subduction-zone mantle: Evidence from the Troodos ophiolite, Cyprus

Geochemical data (including rare-earth elements [REE]) are presented for the primitive (high-MgO) glassy lavas from the upper sections of the Troodos ophiolite. Al2O3/TiO2 ratios are used as an index of degrees of mantle melting and/or successive depletion events. The sources of the lavas have undergone both a ubiquitous introduction of low field-strength elements and a selective addition of light REE, suggesting a two-stage mantle contamination model. Correlations between Al2O3/TiO2 ratios (= depletion index) and the amount of light-REE enrichment present in the lavas suggest a component induced melting model. In this model, progressive addition of a light-REE component induced higher degree batch melts, or enabled melting of highly refractory source material. The light-REE-enrichment event is shown to be of the same character across the ophiolite, but it is limited to the older ocean crust and a transform-influenced regime.

Mantle contamination under Akagi Volcano, Japan, as inferred from combined Sr‐O isotope relationships

The Northeast Japan arc is associated with westward subduction of the Pacific plate beneath the Eurasian plate. At the southernmost region of the arc, the Philippine Sea plate subducts northward into the mantle wedge between the Eurasian plate and the Pacific slab. Akagi volcano located at this region yields volcanic rocks with combined Sr‐O isotope relationships which are explained in terms of the mixing of the mantle material from the mantle wedge and deep ocean and/or continent derived sediments conveyed with the subducting Philippine Sea plate. This implys that the Philippine Sea plate continues to the location just under Akagi volcano, although the seismic Philippine Sea plate does not reach just under the volcano.

Orogenic lherzolite complexes studied by 87Rb- 87Sr: A clue to understand the mantle convection processes?

Ultramafic materials such as lherzolites found in orogenic lherzolite bodies are suitable materials to investigate mantle processes. We present here an extensive K, Rb, Sr and Sr isotopic study on 70 samples, whole rocks and diopsides, for lherzolites and pyroxenites from four bodies of orogenic lherzolites in the Western Mediterranean: Lherz, Freychinède, Lanzo, Beni Bousera. The more striking feature is the complexity of the data which cannot be explained by simple models of partial melting and (or) contamination of homogenous mantle. The scatter of the results of whole rocks may partly be due to further contamination, especially in some pyroxenites. Lherzolite diopsides show great heterogeneities on a small scale. At least two partial melting events separated by about 109 years are required to create these heterogeneities. K, Rb, Sr concentrations and Rb/Sr ratios confirm the residual character of the lherzolites and show that pyroxenites also often have a residual (or cumulate) character, and cannot be direct partial melting products of the lherzolites. Orogenic lherzolites bodies are interpreted as “sandwiches” of slightly depleted mantle parts of different origins coupled together as sheets by mechanical stirring in the convective cell and layered by injected partial melting liquids. In this model, pyroxenitees may have had different histories, for example some may have suffered selective partial melting whereas others have not. This model implies that the oceanic mantle is progressively more and more depleted and that it is mixed, and recycled. It is opposed to models of irreversible differentiation of the mantle, without any recycling of the differentiated oceanic crust, and to models of “catastrophic” event of differentiation. This model also implies that dating such orogenic lherzolite bodies is meaningless.

Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington

Research Article| May 01, 1976 Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington IAN McDOUGALL IAN McDOUGALL 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (5): 777–792. https://doi.org/10.1130/0016-7606(1976)87<777:GAOOBO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation IAN McDOUGALL; Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington. GSA Bulletin 1976;; 87 (5): 777–792. doi: https://doi.org/10.1130/0016-7606(1976)87<777:GAOOBO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Strontium isotope and major- and trace-element data from the voluminous middle Miocene tholeiitic basalts of the Columbia River Group differentiate the lavas stratigraphically and reflect the origin of the basalt. The Picture Gorge Basalt has relatively low Sr87/Sr86 ratios (0.7035 to 0.7039) and a major- and trace-element composition consistent with an origin from the upper mantle. The Picture Gorge Basalt is clearly distinguished from the Imnaha basalt in northeast Oregon and from the widespread, mainly younger Yakima Basalt. The Yakima Basalt, including lava flows in the Vantage, Grande Ronde, and Imnaha regions, forms a relatively coherent geochemical group with moderate to high initial Sr87/Sr86 ratios (0.7045 to 0.7080), which increase with decreasing age of the lava and show an inverse relation with Sr abundance. Possible explanations include progressive contamination of parental mantle—derived magmas by crustal material or derivation from an inhomogeneous upper mantle.It is suggested that the Yakima Basalt formed during a major readjustment episode related to plate motions in an extensional tectonic regime. This caused diapiric upwelling of upper mantle peridotite with formation of great volumes of olivine tholeiite magma, which underwent extensive olivine fractionation. The Columbia Plateau may be a continental analogue of the marginal seas of the western Pacific Ocean. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Possible age of crystallization of pyrope from the Stockdale kimberlite, Kansas, U. S. A.

A possible age of 745 ± 100m.y. (using 50b.y. = half life of 87Rb) has been determined for pyrope from the Stockdale kimberlite assuming an upper mantle initial 87Sr/86Sr ratio of 0.702 ± 0.001. Despite the large error associated with this age it clearly demonstrates that the pyrope crystallized long before its final emplacement which is set at ≤ 100 m.y. Allowance for crustal and mantle contamination and other sources of error have been carefully considered in this determination.