scholarly journals From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 396 ◽  
Author(s):  
Marina Gornova ◽  
Anas Karimov ◽  
Sergei Skuzovatov ◽  
Vasiliy Belyaev
Keyword(s):  
Partial Melting ◽  
Mantle Wedge ◽  
Spreading Center ◽  
Accretionary Complex ◽  
Depleted Mantle ◽  
Basaltic Melt ◽  
Arc Setting ◽  
Late Neoproterozoic ◽  
Back Arc ◽  
Ree Patterns

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N > 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Geochemical and Nd isotopic evolution of Paleoproterozoic arc-type granitoid magmatism in the Flin Flon Belt, Trans-Hudson orogen, Canada

1999 ◽  
Vol 36 (2) ◽  
pp. 227-250 ◽  
Author(s):  
Joseph B Whalen ◽  
Eric C Syme ◽  
Richard A Stern
Keyword(s):  
Temporal Variations ◽  
Granitoid Magmatism ◽  
Early Juvenile ◽  
Depleted Mantle ◽  
Isotopic Evolution ◽  
Basin Formation ◽  
Flin Flon ◽  
Arc Setting ◽  
Back Arc ◽  
Ree Patterns

Granitoid magmatism spans three Flin Flon Belt evolutionary stages: (i) "evolved" arc (~1920 Ma) plus early juvenile arc (1904-1880 Ma) plutonism during intraoceanic arc-back-arc formation; (ii) early (1878-1860 Ma) and middle (1860-1844 Ma) successor arc plutonism following accretion and successor arc(s) development and; (iii) late (1843-1826 Ma) successor arc plutonism accompanying successor basin formation and waning arc magmatism. Amphibole-bearing mineralogy, metaluminous compositions, and igneous microgranitoid enclaves indicate derivation from infracrustal sources. Predominance of intermediate calc-alkaline compositions and negative Nb anomalies on normalized patterns over a 46-77 wt.% silica range indicate an arc setting. Basaltic end members indicate important contributions directly from the mantle. εNd(T) values are predominately in the range 0 to +4.3, reflecting mixing between depleted mantle melts and an Archean crustal component preserved in evolved arc plutons (-3.9 to -6). Temporal variations include the following: (i) early juvenile arc plutons are low K, high field strength element (HFSE) depleted, with relatively flat rare earth element (REE) patterns and negative Eu anomalies, indicative of low-pressure partial melting - fractionation in the mantle wedge, with residual pyroxene and plagioclase; (ii) early and middle successor arc plutonism is medium K, with steep REE patterns and no Eu anomalies, indicative of input from melting of basaltic sources (likely subducted back-arc oceanic crust) under high-pressure conditions with residual garnet and (or) amphibole and no plagioclase; (iii) late successor arc plutons are high K, more HFSE enriched, with both variable REE pattern slopes and Eu anomalies, indicative of a significant petrogenetic role of recycling of preexisting juvenile arc - accretionary complex crust.

scholarly journals Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

Solid Earth ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 23-36
Author(s):  
Tomoaki Morishita ◽  
Naoto Hirano ◽  
Hirochika Sumino ◽  
Hiroshi Sato ◽  
Tomoyuki Shibata ◽  
...  
Keyword(s):  
Partial Melting ◽  
Sea Of Japan ◽  
Alkali Basalt ◽  
Spinel Peridotite ◽  
The Sea Of Japan ◽  
Low Degree ◽  
Tsushima Basin ◽  
Arc Setting ◽  
Element Enrichment ◽  
Back Arc

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

Geodynamic evolution of the southern Abitibi and Pontiac terranes: evidence from geochemistry of granitoid magma series (2700–2630 Ma)

1992 ◽  
Vol 29 (10) ◽  
pp. 2266-2286 ◽  
Author(s):  
Rui Feng ◽  
Rob Kerrich
Keyword(s):  
Rare Earth ◽  
Partial Melting ◽  
Alkali Feldspar ◽  
Quartz Syenite ◽  
Southern Volcanic Zone ◽  
Suprasubduction Zone ◽  
Depleted Mantle ◽  
Quartz Monzonite ◽  
Ree Patterns ◽  
Low K

Four distinct granitic series developed during the evolution of the Archean Abitibi Southern Volcanic Zone (SVZ): (1) A minor synvolcanic tonalite–trondhjemite–granodiorite (TTG) series was emplaced at ~2700 Ma, which has extremely low K2O (<1.5 wt.%), Rb (<50 ppm), and Sr (<200 ppm), higher Nb, Y, and Sc, flat rare-earth-element (REE) patterns, and negative Eu anomalies. (2) A voluminous tonalite – granodiorite – granite – quartz monzonite (TGGM) series developed syntectonically at ~2695–2685 Ma, and displays low K2O (1–3 wt.%), Rb (10–50 ppm), Ba (<1000 ppm), and U, enhanced Sr, depletion of Ta, Nb, and Ti, and strongly fractionated REE patterns (La/Ybn = 49–21). (3) A late-tectonic quartz syenite – quartz monzonite – granite (SMG) series was emplaced from ~2685 to 2675 Ma, and is grossly similar to the TGGM but has lower CaO/(K2O + Na2O) and greater concentrations of Rb, Ba, Th, and U. (4) A late-tectonic to posttectonic alkali feldspar syenite – alkali feldspar quartz syenite (SS) series was emplaced from 2680 to 2670 Ma, and occurs along regional strike-slip structures. The primitive rocks (SiO2 ≤ 65 wt.%) exhibit coenrichment of large-ion lithophile elements (LILE) and mafic elements (Cr, Co, and Ni) and strongly fractionated REE patterns, whereas evolved phases (SiO2 ≥ 65 wt.%) display lower contents of compatible and incompatible elements stemming from differentiation.In the adjacent Pontiac Subprovince and the Lacorne block within the SVZ, two granitic series predominate: (1) A syntectonic to late-tectonic monzodiorite–monzonite–granodiorite–syenite (MMGS) series (2690–2670 Ma) is comparable to the Abitibi SMG and SS series in most major-element, LILE, and REE contents but is distinguished by high MgO contents, extremely high Ba/Th ratios, and coenrichment of Cr, Co, and Ni with light rare-earth elements (LREE), Li, and Cs. (2) A garnet–muscovite–granite (GMG) series (2644 ± 13 Ma) displays K2O/Na2O ≥ 1, restricted SiO2 range (69–75 wt.%), pronounced enrichments of Rb, Li, Cs, Ta, Nb, Th, and U, and moderately fractionated REE's (La/Ybn = 16–0.9), with prominent negative Eu anomalies.The synvolcanic TTG series is interpreted to have formed by differentiation of low-K mafic magmas of the Blake River Group type in suprasubduction-zone environments. Geochemical compositions of the TGGM, SGM, SS, and MMGS series resemble those of Phanerozoic granitoids in island-arc settings and reflect a transition from partial melting of the subducted or subcreted slab to melting of the metasomatized depleted mantle wedge assisted by LILE- and LREE-enriched fluids released from the slab. The GMG, which formed by partial melting of the Pontiac metasediments when the Pontiac Sub-province collided with and underthrust the Abitibi SVZ at ~2670 Ma, is similar to Himalayan collisional leucogranites.

scholarly journals Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

2012 ◽  
Vol 2012 ◽  
pp. 1-18 ◽  
Author(s):  
Charles W. Messo ◽  
Shukrani Manya ◽  
Makenya A. H. Maboko
Keyword(s):  
Partial Melting ◽  
Greenstone Belt ◽  
Mantle Wedge ◽  
Spatial Association ◽  
Volcanic Rocks ◽  
Low Concentrations ◽  
Peridotite Mantle ◽  
Ree Patterns ◽  
Subducting Slab ◽  
Oceanic Slab

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

scholarly journals Evolution of Subduction Dynamics beneath West Avalonia in Middle to Late Ordovician Times

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
pp. 1-22
Author(s):  
Pierre Jutras ◽  
J. Brendan Murphy ◽  
Dennis Quick ◽  
Jaroslav Dostal
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Volcanic Rocks ◽  
Upper Ordovician ◽  
The North ◽  
Trace Element Contents ◽  
Element Contents ◽  
Arc Setting ◽  
Back Arc ◽  
High Degree

Abstract Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

2007 ◽  
Vol 144 (2) ◽  
pp. 305-317 ◽  
Author(s):  
QIUGEN LI ◽  
SHUWEN LIU ◽  
ZONGQI WANG ◽  
QUANREN YAN ◽  
ZHAOJIE GUO ◽  
...  
Keyword(s):  
Partial Melting ◽  
Minor Amount ◽  
Tectonic Zone ◽  
Depleted Mantle ◽  
Eastern Segment ◽  
Tectonic Zones ◽  
Granitoid Gneisses ◽  
A Minor ◽  
T Values ◽  
Ree Patterns

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce (<5.3) and Sr/Y (<28), relatively high Mg no. (51.0–57.0), K2O (2.65–4.04 wt %) contents and εNd(t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K2O contents, low Al2O3/(FeO* + MgO + TiO2) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low εNd(t) values and older TDM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

scholarly journals Fault-controlled deep hydrothermal flow in a back-arc tectonic setting, SE Tyrrhenian Sea

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Maria Filomena Loreto ◽  
Doğa Düşünür-Doğan ◽  
Serkan Üner ◽  
Yeliz İşcan-Alp ◽  
Neslihan Ocakoğlu ◽  
...  
Keyword(s):  
Partial Melting ◽  
Mantle Wedge ◽  
Tectonic Setting ◽  
Geothermal Gradient ◽  
Tyrrhenian Sea ◽  
Hydrothermal Circulation ◽  
Magmatic Intrusions ◽  
Back Arc ◽  
Structural High ◽  
Hydrothermal Flow

AbstractUnderstanding magmatic systems and deep hydrothermal circulation beneath arc-volcanoes provides insights into deep processes associated with slab-subduction and mantle-wedge partial melting. Here we analyze hydrothermal flow below a structural high (Capo Vaticano Ridge, CVR) located offshore Capo Vaticano (western Calabria) and affected by magmatic intrusions generated from above the Ionian subducting-slab. In order to explain observations, we combine geophysical and numerical modelling results. Fluid-flow modelling shows that temperature distribution and geothermal gradient are controlled mainly by hydrothermal circulation, in turn affected by heat source, fault pattern, rock permeability, basement topography and sediment thickness. Two main faults, shaping the structural high and fracturing intensely the continental crust, enable deep hydrothermal circulation and shallow fluid discharge. Distribution of seismicity at depth supports the hypothesis of a slab below Capo Vaticano, deep enough to enable mantle-wedge partial melting above the subduction zone. Melt migration at shallow levels forms the magmatic intrusions inferred by magnetic anomalies and by δ3He enrichment in the discharged fluids at the CVR summit. Our results add new insights on the southern Tyrrhenian Sea arc-related magmatism and on the Calabrian inner-arc tectonic setting dissected by seismogenic faults able to trigger high-destructive earthquakes.

scholarly journals Geochronology and Geochemistry of the Karadaban Bimodal Volcanic Rocks in the Altyn Area, Xinjiang: Implications for the Tectonic Evolution of the Altyn Ocean

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-25
Author(s):  
Wen-Bin Jia ◽  
Guang-Sheng Yan ◽  
Xiao-Fei Yu ◽  
Yong-Sheng Li ◽  
Sandro Conticelli ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Bimodal Distribution ◽  
Oceanic Lithosphere ◽  
Isotopic Signature ◽  
Icp Ms ◽  
Depleted Mantle ◽  
Back Arc ◽  
Ree Patterns ◽  
Light Rare Earth Elements ◽  
Felsic Rocks

Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA-ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial87Sr/86Sr=0.70413–0.70817,εNdt=2.7to 3.7) suggest that they were likely derived from a depleted mantle source but mixed with crustal components while upwelling. The felsic rocks show an A-type affinity, with high alkalis and Rb/Sr and Ga/Al ratios; enriched in LILE (e.g., Rb, K, Th, U, and REE) and depleted in Ba, Sr, Nb, P, and Ti; and with fractionated REE patterns with strong negative Eu anomalies. The combination of the decoupling ofεNdtvalues (−2.5 to −6.3) andεHftvalues (+5.5 to +14.7) in the setting of subduction indicates that the felsic rocks were generated by partial melting of the juvenile crustal as a result of magma upwelling. The geochemical and Sr-Nd-Hf isotopic characteristics, coupled with regional geology, indicate that the formation of the Karadaban bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the back arc in response to the retreat of the subducting northern Altyn oceanic lithosphere may account for the Karadaban bimodal volcanic rocks.

Crustal assimilation in the Burnt Lake metavolcanics, Grenville Province, southeastern Ontario, and its tectonic significance

1997 ◽  
Vol 34 (9) ◽  
pp. 1272-1285 ◽  
Author(s):  
T. E. Smith ◽  
P. E. Holm ◽  
N. M. Dennison ◽  
M. J. Harris
Keyword(s):  
Rare Earth ◽  
Trace Element ◽  
Partial Melting ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Volcanic Rocks ◽  
Lake Area ◽  
Depleted Mantle ◽  
Mafic Magmas ◽  
Back Arc

Three intimately interbedded suites of volcanic rocks are identified geochemically in the Burnt Lake area of the Belmont Domain in the Central Metasedimentary Belt, and their petrogenesis is evaluated. The Burnt Lake back-arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back-arc basins formed in continental crust. The Burnt Lake continental tholeiitic suite comprises basalts and andésites similar in trace element composition to continental tholeiitic sequences. The Burnt Lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts. Rare earth element models suggest that the Burnt Lake back-arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5% partial melting of an amphibole-bearing depleted mantle, enriched in light rare earth elements by a subduction component. The modelling also suggests that the Burnt Lake continental tholeiitic rocks were formed by contamination – fractional crystallization of mixtures of mafic magmas, derived by ~3% partial melting of the subduction-modified source, and rhyolitic crustal melts. These models are consistent with the suggestion that the Belmont Domain of the Central Metasedimentary Belt formed as a back-arc basin by attenuation of preexisting continental crust above a westerly dipping subduction zone.

scholarly journals Mantle Evolution from Ocean to Arc: The Record in Spinel Peridotite Xenoliths in Mt. Pinatubo, Philippines

Minerals ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 515 ◽  
Author(s):  
Betchaida Payot ◽  
Shoji Arai ◽  
Masako Yoshikawa ◽  
Akihiro Tamura ◽  
Mitsuru Okuno ◽  
...  
Keyword(s):  
Partial Melting ◽  
Earth Element ◽  
Mantle Wedge ◽  
Chromian Spinel ◽  
Spinel Peridotite ◽  
Spinel Lherzolite ◽  
Mantle Evolution ◽  
Peridotite Xenoliths ◽  
Ree Patterns ◽  
Subsequent Modification

A suite of peridotite xenoliths were collected from lahar flow deposits located close to the summit of Mt. Pinatubo. Spinel harzburgite is the most dominant lithology among dunites, pyroxenites and websterites. A rare spinel lherzolite xenolith (P12-7) is also present in this suite. The spinel lherzolite has well-preserved protogranular texture with very minimal presence of secondary amphibole, low Cr# in the chromian spinel, and depleted and hump shaped patterns of chondrite-normalized rare earth element (REE) patterns for the clinopyroxenes. In contrast, the spinel harzburgites contain abundant secondary amphiboles and orthopyroxenes, higher Cr# in the spinel, and slightly elevated patterns for the chondrite-normalized REE patterns for the amphiboles. The spinel lherzolite also exhibits higher olivine Fo content for a given spinel Cr# compared to the spinel harzburgites. The spinel lherzolite is interpreted as a typical residue from partial melting of abyssal peridotites whereas the spinel harzburgites may have formed via partial melting with subsequent modification during the influx of fluids in the mantle wedge. Our results suggest that fragments of MOR-derived lithosphere exist in the mantle wedge beneath the Philippine island arc. This work provides evidence for the conversion of abyssal to arc peridotites in the mantle wedge.

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