scholarly journals U–Pb Zircon Geochronological and Petrologic Constraints on the Post-Collisional Variscan Volcanism of the Tiddas-Souk Es-Sebt des Aït Ikko Basin (Western Meseta, Morocco)

Minerals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1099
Author(s):  
Ismail Hadimi ◽  
Nasrrddine Youbi ◽  
Abdelhak Ait Lahna ◽  
Mohamed Khalil Bensalah ◽  
Oussama Moutbir ◽  
...  
Keyword(s):  
Crustal Contamination ◽  
Mafic Magma ◽  
Variscan Orogeny ◽  
Calc Alkaline ◽  
Specific Chemical ◽  
The Third ◽  
Mineralogical Characteristics ◽  
Eruptive Cycle ◽  
Ash Formation ◽  
Volcanic Succession

The NE–SW trending Tiddas Souk Es-Sebt des Ait Ikko (TSESDAI) basin, located at 110 km southeast of Rabat, in the region of Khmesset between the village of Tiddas Souk Es-Sebt des Ait Ikko, is the third largest late Palaeozoic continental trough in the northern Central Moroccan Meseta. It is a ~20 km long and ~2–3 km wide basin, comprising mainly mixed volcano-sedimentary reddish-purple continental Permian rocks laying with an angular unconformity on Visean deep marine siliciclastic sediments and unconformably overlain by the Triassic and Cenozoic formations. In this study we aim to better determine the age of Permian volcanics and their chemical and mineralogical characteristics, as well as assess the provenance of inherited zircons, thus contributing to the understanding of the late stages of the Variscan orogeny in Morocco. The standard volcanic succession includes the following terms: (i) andesites, lapilli tuffs and andesitic ash deposits; (ii) accumulations of rhyolitic lavas; (iii) lapilli tuffs and rhyolitic ash (formation F1); (iv) flows and breccias of dacites; (v) andesite flows; and (vi) basaltic flows. The various volcanic and subvolcanic studied rocks display calc-alkaline-series characteristics with high contents of SiO2, Al2O3, CaO, MgO, and relatively abundant alkalis, and low contents of MnO. In the classification diagram, the studied facies occupy the fields of andesites, trachy-basalts, dacites, trachydacites, and rhyolites and display a sub-alkaline behavior. These lavas would be derived from a parental mafic magma (basalts) produced by partial fusion of the upper mantle. Specific chemical analyses that were carried out on the mineralogical phases (biotite and pyroxene) revealed that the examined biotites can be classified as magnesian and share similarities with the calc-alkaline association-field, while the clinopyroxenes are mainly augites and plot on the calc-alkaline orogenic basalt field. Andesites and dacites of TSESDAI show similarities with the rocks of the calc-alkaline series not linked to active subduction and which involve a continental crust in their genesis. The existence of enclaves in the lavas of the TSESDAI massif; the abnormally high contents of Rb, Ba, Th, and La; and the systematic anomalies in TiO2 and P2O5 indicate also a crustal contamination mechanism. Three magmatic episodes are distinguished with two episodes that correspond to an eruptive cycle of calc-alkaline andesites and rhyolites followed by a basaltic episode. The SHRIMP U–Pb geochronologic data of zircons recovered from the rhyolite dome of Ari El Mahsar in TSESDAI basin show a Concordia age of 286.4 ± 4.7 Ma interpreted to date the magmatic crystallization of this dome. Thus, the rhyolite likely belongs to the third magmatic episodes of TSESDAI.

Rare earth element concentrations in Quaternary volcanic rocks of southwestern British Columbia

1984 ◽  
Vol 21 (6) ◽  
pp. 731-736 ◽  
Author(s):  
Nathan L. Green ◽  
Paul Henderson
Keyword(s):  
British Columbia ◽  
Earth Element ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Lake Area ◽  
Calc Alkaline ◽  
Lower Light ◽  
The Individual ◽  
High Level ◽  
Ree Patterns

A suite of hy-normative hawaiites, ne-normative mugearite, and calc-alkaline andesitic rocks from the Garibaldi Lake area exhibits fractionated, slightly concave-upward REE patterns (CeN/YbN = 4.5–15), heavy REE contents about 5–10 times the chondritic abundances, and no Eu anomalies. It is unlikely that the REE patterns provide information concerning partial melting conditions beneath southwestern British Columbia because they have probably been modified substantially by upper crustal processes including crustal contamination and (or) crystal fractionation. The REE contents of the Garibaldi Lake lavas are not incompatible with previous interpretations that (1) the hawaiites have undergone considerable fractionation of olivine, plagioclase, and clinopyroxene; and (2) the individual andesitic suites were derived from separate batches of chemically distinct magma that evolved along different high-level crystallization trends. In general, however, the andesites are characterized by lower light REE contents than the basaltic andesites. These differences in LREE abundances may reflect different amounts of LREE-rich accessory phases, such as apatite, sphene, or allanite, assimilated from the underlying quartz diorites.

Petrographic, Geochemical and Isotopic (Sr–Nd–Pb–Os) Study of Plio-Quaternary Volcanics and the Tertiary Basement in the Jorullo-Tacámbaro Area, Michoacán-Guanajuato Volcanic Field, Mexico

2019 ◽  
Vol 60 (12) ◽  
pp. 2317-2338 ◽  
Author(s):  
Marie-Noëlle Guilbaud ◽  
Claus Siebe ◽  
Christine Rasoazanamparany ◽  
Elisabeth Widom ◽  
Sergio Salinas ◽  
...  
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Volcanic Belt ◽  
Volcanic Field ◽  
Calc Alkaline ◽  
Trace Element Pattern ◽  
Alkaline Rocks ◽  
Rock Types ◽  
Oceanic Sediments ◽  
Element Pattern

Abstract The origin of the large diversity of rock types erupted along the subduction-related Trans-Mexican Volcanic Belt (TMVB) remains highly debated. In particular, several hypotheses have been proposed to explain the contemporary eruption of calc-alkaline and alkaline magmas along the belt. The Michoacán-Guanajuato Volcanic Field (MGVF) is an atypical, vast region of monogenetic activity located in the western-central part of the TMVB. Here we present new petrographic, geochemical, and isotopic (Sr–Nd–Pb–Os) data on recent volcanics in the Jorullo-Tacámbaro area that is the closest to the oceanic trench. TMVB-related volcanics in this area are Plio-Quaternary (<5 Ma) and mainly form a calc-alkaline series from basalts to dacites, with rare (<5 vol. %) alkaline rocks that range from trachybasalts to trachydacites, and transitional samples. Crystal textures are consistent with rapid crystallization at shallow depth and processes of mixing of similar magma batches (magma recharge). All of the samples exhibit an arc-type trace element pattern. Alkaline and transitional magmas have higher Na2O and K2O, lower Al2O3, and higher concentrations in incompatible elements (e.g. Sr, K, Ba, Th, Ce, P) compared to calc-alkaline rocks. Calc-alkaline rocks are similar isotopically to transitional and alkaline samples, except for a few low 87Sr/86Sr samples. Sr, Nd and Pb isotopes do not correlate with MgO or 187Os/188Os, indicating that they were not significantly influenced by crustal contamination. Isotopic and trace-element systematics suggest that the Tacámbaro magmas are produced by melting of a mantle wedge fluxed by fluids derived from a mixture of subducted sediments and altered oceanic crust. Alkaline and transitional magmas can be derived from a lower degree of partial melting of a similar source to that of the calc-alkaline rocks, whereas the few low 87Sr/86Sr calc-alkaline rocks require a lower proportion of fluid derived from oceanic sediments and crust. Volcanism at the trenchward edge of the MGVF was thus driven purely by subduction during the last 5 Ma, hence discarding slab rollback in this sector of the TMVB.

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

Lithos ◽  
1999 ◽  
Vol 46 (4) ◽  
pp. 773-802 ◽  
Author(s):  
R. Benito ◽  
J. López-Ruiz ◽  
J.M. Cebriá ◽  
J. Hertogen ◽  
M. Doblas ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Calc Alkaline ◽  
Se Spain ◽  
High K ◽  
O Isotope

Development of the New Hebrides archipelago

1975 ◽  
Vol 272 (918) ◽  
pp. 277-285 ◽  
Keyword(s):  
Late Miocene ◽  
Middle Miocene ◽  
First Episode ◽  
Late Oligocene ◽  
Calc Alkaline ◽  
Land Area ◽  
Early Pliocene ◽  
The Third ◽  
New Hebrides ◽  
Volcanic Islands

The general geology of the New Hebrides is summarized in terms of three volcanic and two main sedimentary episodes. Calc-alkaline volcanics ol the first episode occur on the western islands and accumulated mainly on the submarine slopes below small reef-fringed volcanic islands in Late Oligocene to Middle Miocene times. During the Late Miocene and Early Pliocene wholly submarine tholeiitic or high-Al volcanics accumulated in the eastern and southern part of the New Hebrides while calcareous sediments were forming in the western islands. During the third volcanic phase, of Pliocene to Recent age, regional uplift has led to most of the volcanics being subaerial while extensive flights of limestone terraces occur round the older islands. In consequence the land area of the New Hebrides has increased rapidly during Quaternary times. The landforms produced are briefly described.

scholarly journals Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

1989 ◽  
Vol 157 ◽  
pp. 1-164
Author(s):  
L.M Larsen ◽  
W.S Watt ◽  
M Watt
Keyword(s):  
North Atlantic ◽  
Mantle Source ◽  
Atlantic Coast ◽  
Crustal Contamination ◽  
Basalt Magma ◽  
Magma Chambers ◽  
East Greenland ◽  
The Third ◽  
The North ◽  
The North Atlantic

The early Tertiary plateau basalts in East Greenland are situated on a continental margin and were erupted during continental break-up and initiation of sea-floor spreading in the North Atlantic. In the region stretching from Scoresby Sund southward to 69°N 40 000 km2of basalts with an average thickness of 1.5 km have been investigated by measuring and flow-to-flow sampling of 130 profiles, followed by major element geochemical analysis and microprobe analysis, trace element analysis and some Sr isotope data. The basalts rest on Mesozoic sediments in the east and on Precambrian gneiss in the west. Six basalt formations are defined: the Magga Dan, Milne Land and Geikie Plateau Formations form a lower regional sequence erupted in one volcanic episode from sites in the NW part of the region; the Rømer Fjord and Skrænterne Formations form an upper regional sequence erupted in a subsequent volcanic episode in which eruption sites moved SE to centres east of the present Atlantic coast; the Igtertivâ Formation and a coast-parallel dyke swarm formed in a third volcanic episode only recorded at the Atlantic coast. The lavas are essentially flat-lying; a narrow strip along the Atlantic coast is extensively block faulted. Single lava flows are extensive (max. 11 000 km2) and voluminous (max. 300 km3). They are well preserved, with metamorphism of the low zeolite facies. All the lavas and most of the dykes are fractionated tholeiitic basalts with Mg/(Mg+Fe2+) ratios of 0.66-0.39 and TiO2 = 1.2-4.5%. The major part (the 'main basalts', 96% by volume) have Mg ratios of 0.56-0.39, while only 4 vol.% are Mg-rich basalts with Mg ratios of 0.66-0.57. A nephelinitic tuff layer occurs at the base of the second sequence. A few dykes are alkaline. The Mg-rich basalts have microphenocrysts of olivine (FO90-70) and chromite, while the main basalts comprise both aphyric and porphyritic sequences. Phenocrysts of plagioclase (An88-37) are abundant, of olivine (FO80-57) are sparse but ubiquitous, and of augite (FS9-20) sparse and often absent. Groundmass phases are olivine (to FO3737), plagioclase (to An13, augite (to FS62), pigeonite (Fs26-50), titanomagnetite and ilmenite. All rocks contain several per cent fine-grained mesostasis. The phenocrysts frequently show disequilibrium textures and a wide range of compositions within one sample. Extrusion temperatures are calculated to 1280-1110°C, and densities to 2.68-2.78 g/cm3, increasing with fractionation. The volcanic episodes are demonstrated in systematic compositional variations with height in the basalt sequence. Each of the two major episodes started with a variety of lava compositions including Mg-rich basalts, followed by a thick sequence of 'main basalts' showing a systematic decrease of TiO2 and other incompatible elements with height, and ending with a reversal to higher TiO2 values. The third episode is not cyclic, and its products have changed incompatible element ratios. The Mg-rich basalts comprise depleted MORB type basalts, relatively enriched olivine tholeiites, and very enriched tholeiites (Mikis type basalt). Sr isotopes show 87Sr/86Sr ratios of 0.7034 in most basalts and 0.7045 in the Mikis type basalt, while some Si-rich basalts have ratios up to 0.7079. The East Greenland basalts are 'initial rifting' basalts very similar to those in Deccan. The magmas have equilibrated at low pressures in crustal magma chambers. The main basalts have fractionated ol + pl + cpx no matter whether they are aphyric or porphyritic. Simple crystal fractionation can account for sub-trends but not for the complete compositional variation of the main basalts. This is considered as resulting from fractionation in open magma chambers which were repeatedly filled, mixed and tapped. The decrease in TiO2 with height in each volcanic episode indicates increasing magma input rate and shorter residence time in the chamber, while the final reversal indicates the decline and cessation of activity. There is evidence for widespread crustal contamination (1-4%) in the magma chambers of the two lowest formations. Crustal contamination of magmas on the way to the surface occurred sporadically throughout both sequences. One case of magma mixing occurred when a Mg-rich basalt magma invaded the regional main basalt magma chamber. The Mg-rich basalts cannot be directly related to each other or to the main basalts. A petrogenetic scheme is suggested where the Mikis type basalt originated in, or contains an addition from, an undepleted or enriched mantle source. All the other magma types originated in a depleted mantle source by varying degrees and possibly depths of melting. Increasing degrees of melting are indicated for the types nephelinite - enriched olivine tholeiite – main basalt parent – MORB type basalt. The MORB type basalt may also be produced by melting of a residuum. The basalts of the third volcanic episode include another component of mantle or basaltic crust. The three recorded volcanic episodes are related to rifting events during the break-up of the North Atlantic continent, viewed as repeated attempts to straighten out a bend in the original line of opening. The two first rifting events failed while the third for a short while produced oceanic crust. Compared to other regions of the North Atlantic volcanic province the Scoresby Sund basalts are similar to basalts from Kangerdlugssuaq, northern East Greenland, West Greenland, the Faeroes, the Vøring Plateau and some basalts on lceland. The main magma source for the North Atlantic province was similar to that of the lceland hotspot, but enriched subcontinental lithosphere may also have participated in the stage of initial rifting. A correlation for the volcanic episodes throughout East Greenland and the Faeroes is proposed.

scholarly journals Lithostratigraphy, geology and geochemistry of the volcanic rocks of the Vaigat Formation on Disko and Nuussuaq, Paleocene of West Greenland

2017 ◽  
Vol 39 ◽  
pp. 1-244
Author(s):  
Asger Ken Pedersen ◽  
Lotte Melchior Larsen ◽  
Gunver Krarup Pedersen
Keyword(s):  
Volcanic Rocks ◽  
First Episode ◽  
The Third ◽  
The North ◽  
Rock Types ◽  
Magma Reservoirs ◽  
Native Iron ◽  
Volcanic Succession ◽  
High Level ◽  
Contamination Events

The Paleocene volcanic rocks in the Nuussuaq Basin on Disko and Nuussuaq comprise the Vaigat Formation (c. 62–61 Ma) and the Maligât Formation (c. 60 Ma). The Vaigat Formation in this area is 0–1600 m thick and is dominated by olivine-rich picrites. The formation was deposited during three volcanic episodes and is divided into 10 formally defined members and about 20 informal units. The first episode gave rise to the Anaanaa Member. The second episode gave rise to the Naujánguit Member, which is intercalated with the minor, crustally contaminated Nuusap Qaqqarsua, Nuuk Killeq, Asuk, Tunoqqu and Kûgánguaq members and the uncontaminated Qordlortorssuaq Member. The third episode gave rise to the Ordlingassoq Member and the minor alkaline Manîtdlat Member. Contemporaneous sediments deposited during the first two episodes are the marine Eqalulik Formation, and during the third episode the nonmarine Atanikerluk Formation. During the second episode, the polarity of the geomagnetic field changed from normal (Chron C27n) via a transition zone to reversed (C26r). The deposits of the first volcanic episode are situated on western Nuussuaq. During the second and third episodes, the volcanism gradually spread eastwards and southwards so that the Vaigat Formation now forms a domed structure, thickest in the north, thinning out on northern Disko and reaching eastwards to the high gneiss country on central Nuussuaq. The earliest eruptions took place on the sea floor and quickly built up a subaerial lava plateau. All three episodes gave rise to complicated facies changes between subaqueous and subaerial eruption products caused by the eastmoving volcanism, subsidence, volcanic aggradation and blockage of the sea connection against the elevated eastern gneiss country. Eruption sites are widespread for all three volcanic episodes. Within certain time periods, a number of contemporaneous high-level magma reservoirs developed within sediments of the Nuussuaq Group, and the crustally contaminated members formed in these reservoirs by reaction between Mg-rich magmas and sediments. The uncontaminated rocks in the Vaigat Formation are picrites with 12–31 wt% MgO and subordinate basalts with 7–12 wt% MgO. The crustally contaminated rocks range from silicic picrites with 12–16 wt% MgO (Nuusap Qaqqarsua Member) to native-iron-bearing magnesian andesites with 6–10 wt% MgO and up to 62 wt% SiO2 (Asuk Member). The Asuk Member includes unique, strongly reduced rock types with native iron, graphite and sulfide. The contaminated units have individually distinct compositions, indicating individually different contamination events. The alkaline Manîtdlat Member contains an enriched lithospheric component. Present-day seeps of migrated oil are widespread in the oldest part of the volcanic succession on western Nuussuaq. Some of the contaminated magmas in the Asuk and Kûgánguaq members have fractionated sulfides with Cu and Ni and have been explored for nickel and platinum-group elements. 

scholarly journals Comparison of Granitoid Characteristics West Kalimantan and Karangsambung Based On Mineralogical And Geochemical Aspects

2021 ◽  
Vol 6 (3) ◽  
pp. 152-163
Author(s):  
Kevin Setyo Adi Nugroho ◽  
Iwan Setiawan ◽  
Tri Winarno
Keyword(s):  
Fluorescence Analysis ◽  
Calc Alkaline ◽  
The West ◽  
Entire Study ◽  
West Kalimantan ◽  
Rock Samples ◽  
Mineralogical Characteristics ◽  
Tectonic Environment ◽  
High K ◽  
Different Characteristics

Indonesia was included in the ring of fire so that it has various types of tectonic products, one of which is granitoid. Granitoid is  very complex rock and many are found in Indonesia. Some of them are found in West Kalimantan and Karangsambung. Basis of the reasearch is there is no research that compares granitoid in two regions. The purpose of this study was to compare rock characteristics and granite petrogenesis of West Kalimantan and Karangsambung. The research method used was collecting data on field, also laboratory analysis of rock samples using a polarization microscope, refraction microscope, and X-Ray Fluorescence analysis. The mineralogical characteristics of each study area tend to be almost the same. The predominant composition of the main minerals is quartz, plagioclase and orthoclase. But specifically the rock samples from West Kalimantan have been altered from phylic-silicification-propylitic. The entire study area contained accessory minerals, namely apatite, zircon, titanite, and for monazite only in the West Kalimantan sample. There was mineralization up to the supergene stage in the presence of the characteristic minerals for the supergene covelite and chalcocytes in the West Kalimantan sample. Geochemical analysis of both regions shows the same magma affinity, namely Calc Alkaline - High K Calc Alkaline. For West Kalimantan, the value of A / CNK <1.1 has a type metaluminious and > 1.1 a type peraluminious. Meanwhile, Karangsambung A / CNK value <1.1 has a type metaluminious. So that West Kalimantan granite has two I-type and S-type. While Karangsambung is I-type. West Kalimantan granite is formed in continental arc granite (CAG) and continental collision granite (CCG). Meanwhile, Karangsambung in Volcanic Arc Granite (VAG). It can be concluded that the granites of the two regions have quite different characteristics even though they belong to a relatively similar tectonic environment.

scholarly journals Study of Petrogenesis and Tectonomagmatic Environments of Eocene Calc-Alkaline Volcanic Units in South and Southeast of Beroni Village in Southwest Ardestan (Isfahan)

2015 ◽  
Vol 10 (Special-Issue1) ◽  
pp. 719-726
Author(s):  
Sayyed Roshan ◽  
Ali Khan Nasr Esfahani
Keyword(s):  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Hydrothermal Solution ◽  
Hydrothermal Solutions ◽  
Calc Alkaline ◽  
Silica Rocks ◽  
Calcium Magnesium ◽  
Alkaline Magmas ◽  
Host Rocks ◽  
Lateral Segregation

The study area is located in south and southeast of Beroni Village. It contains volcanic rocks including andesitic-basaltic, pyroxene-bearing andesite, andesite, dacite, rhyodacite, rhyolites and Eocene-Oligocene ignimbrites. The volcanic rocks are cut by an intrusive mass with great spreading in the region. According to lithological studies, the calc-alkaline magmas in continental margin arcs are comprised of mantle and fluid crust. The basic elements in the volcanic rocks were studied in terms of petrological indices. According to the results, the metaluminous rocks underwent crustal contamination. Due to chemical reactions between the hydrothermal solution and volcanic host rocks, hydrothermal solutions in volcanic rocks penetrate the surrounding silica rocks and thus some elements such as zinc and barium diffuse in the rocks. In addition, calcium, magnesium and iron have been drawn inwards from the surrounding rocks causing lateral segregation.

Late Ordovician to early Silurian calc-alkaline magmatism in the Xiemisitai Mountains, northern West Junggar: a response to the subduction of the Junggar-Balkhash Ocean

2019 ◽  
Vol 61 (16) ◽  
pp. 2000-2020 ◽  
Author(s):  
Chen ◽  
Ma ◽  
Simon ◽  
Du ◽  
Han ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Crustal Contamination ◽  
Late Ordovician ◽  
Alkaline Magmatism ◽  
Calc Alkaline ◽  
Hydrous Minerals ◽  
Isotopic Signatures ◽  
Central Asian ◽  
West Junggar ◽  
Early Palaeozoic

The Early Palaeozoic tectonic evolution of northern West Junggar is essential for understanding the tectonic framework and accretionary processes in the southwestern segment of the Central Asian Orogenic Belt. However, its evolutionary history is controversial due to insufficient pre-late Silurian magmatic data. In order to clarify this issue, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses on samples from the Dengdeer and Maodun complexes in the Xiemisitai Mountains of Boshchekul-Chingiz (BC) arc, northern West Junggar. LA-ICP-MS zircon U-Pb dating yielded ages ranging from 445 Ma to 429 Ma, revealing the Late Ordovician to early Silurian magmatic events in northern West Junggar. The dioritic and metaluminous Dengdeer complex has high Mg# (49.5–52.9), moderate La/Yb (11.0–12.3), and low initial 87Sr/86Sr (0.70324–0.70410) and εNd(t) (+3.7 to +4.5). This complex exhibits variable zircon εHf(t) (+11.31 to +17.62), young Hf model ages (470–440 Ma), and appears to have been produced by differentiation of melts from metasomatized mantle. The intermediate to felsic, metaluminous to weakly peraluminous Maodun complex has abundant enclaves and inherited zircons. This complex seems to have fractionated from mantle- and crustal-derived magma mixing with some crustal contamination, based on its high Mg# (36.6–49.5), low Sr/Y (8.7–29.6), Nb/Ta (8.3–13.9), initial 87Sr/86Sr (0.70377) and εNd(t) (+4.0 to +4.2), and variable zircon εHf(t) (+11.59 to +15.21). The Dengdeer and Maodun samples are high-K calc-alkaline, rich in hydrous minerals, Rb, Th, and LREEs, depleted in Nb, Ta, and Ti, and show decoupling Hf-Nd isotopic signatures. These characteristics indicate that their source had been modified by melts from slab sediments. After combining our data with previous observations, we conclude that a trench-ward migration of an arc-back-arc system occurred during the Late Ordovician to early Silurian in northern West Junggar, while the Dengdeer and Maodun complexes were related to subduction-accretionary processes of the Junggar-Balkhash oceanic slab beneath the BC arc.

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