The Border Ranges ultramafic and mafic complex, south-central Alaska: cumulate fractionates of island-arc volcanics

1985 ◽  
Vol 22 (7) ◽  
pp. 1020-1038 ◽  
Author(s):  
Laurel E. Burns
Keyword(s):  
Fractional Crystallization ◽  
Compositional Data ◽  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Island Arc ◽  
Minor Amount ◽  
The North ◽  
South Central ◽  
Mafic Complex ◽  
Wide Range

A discontinuous, elongate zone of mafic and ultramafic plutonic rock crops out in south-central Alaska for a distance of more than 1000 km. Intermediate- and detailed-scale geologic mapping, petrographic study, and compositional data suggest that the plutonic rocks are compositionally, petrologically, and mineralogically distinct from rocks in mid-ocean ridge and back-arc basin ophiolites. The mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc.The mafic–ultramafic zone, referred to as the Border Ranges ultramafic and mafic complex (BRUMC), is composed of ultramafic cumulates, gabbronorite cumulates, and massive gabbronorites. A very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the BRUMC. A thick sequence of andesitic volcanic rocks, the Talkeetna Formation of Early Jurassic age, lies to the north of and structurally above the mafic–ultramafic zone. Voluminous calcalkaline plutons composed of quartz diorite, tonalite, and minor granodiorite intrude both the mafic plutonic complexes and the andesitic volcanic rocks.The cumulate ultramafic sections are largely composed of dunite ± chromite, wehrlite, clinopyroxenite, and websterite and are characterized by a wide range of Mg–Fe silicate compositions (Fo90–81; En45–50, Fs1–7, Wo45–49; En88–82, Fs11–17), chrome-rich spinels, and a lack of plagioclase. The gabbroic sections are composed of gabbronorites with up to 10–15% magnetite ± ilmenite. Hornblende, if present, is a very minor phase in most gabbroic rocks. The coexisting mineral compositions seen in the gabbroic rocks of the BRUMC (relatively iron-rich pyroxene—Fs6–13, En45–40; En81–63 —and calcic plagioclase An75–100) and their association with magnetite are common in plutonic xenoliths in island-arc rocks.The mineralogy and composition of the gabbroic rocks in the BRUMC are consistent with the fractional crystallization products predicted to be associated with the formation of andesite from a basaltic magma. Consideration of additional data, including detailed and regional field mapping of the plutonic and volcanic rocks and geochronology of the BRUMC and the nearby Talkeetna arc volcanic rocks, strongly suggests that the BRUMC represents relatively deep fractional crystallization products of magmas that produced the Talkeetna Formation volcanic rocks. Field relationships also indicate that intrusion of quartz diorites, tonalites, and granodiorites of batholithic proportions occurred slightly later than formation of the BRUMC.

Subduction-related magmatic imprint of most Philippine ophiolites: implications on the early geodynamic evolution of the Philippine archipelago

2004 ◽  
Vol 175 (5) ◽  
pp. 443-460 ◽  
Author(s):  
Rodolfo A. Tamayo* ◽  
René C. Maury* ◽  
Graciano P. Yumul ◽  
Mireille Polvé ◽  
Joseph Cotten ◽  
...  
Keyword(s):  
Partial Melting ◽  
Subduction Zone ◽  
Volcanic Rocks ◽  
The Philippines ◽  
Island Arc ◽  
Island Arcs ◽  
Geodynamic Evolution ◽  
Wide Range ◽  
Opening And Closing ◽  
Back Arc

Abstract The basement complexes of the Philippine archipelago include at least 20 ophiolites and ophiolitic complexes. These complexes are characterised by volcanic sequences displaying geochemical compositions similar to those observed in MORB, transitional MORB-island arc tholeiites and arc volcanic rocks originating from modern Pacific-type oceans, back-arc basins and island arcs. Ocean island basalt-like rocks are rarely encountered in the volcanic sequences. The gabbros from the ophiolites contain clinopyroxenes and plagioclases showing a wide range of XMg and An values, respectively. Some of these gabbros exhibit mineral chemistries suggesting their derivation from basaltic liquids formed from mantle sources that underwent either high degrees of partial melting or several partial melting episodes. Moreover, some of the gabbros display a crystallization sequence where orthopyroxene and clinopyroxene appeared before plagioclase. The major element compositions of coexisting orthopyroxenes and olivines from the mantle peridotites are consistent with low to high degrees of partial melting. Accessory spinels in these peridotites display a wide range of XCr values as well with some of them above the empirical upper limit of 0.6 often observed in most modern mid-oceanic ridge (MOR) mantle rocks. Co-existing olivines and spinels from the peridotites also exhibit compositions suggesting that they lastly equilibrated under oxidizing mantle conditions. The juxtaposition of volcanic rocks showing affinities with modern MOR and island arc environments suggests that most of the volcanic sequences in Philippine ophiolites formed in subduction-related geodynamic settings. Similarly, their associated gabbros and peridotites display mineralogical characteristics and mineral chemistries consistent with their derivation from modern supra-subduction zone-like environments. Alternatively, these rocks could have, in part, evolved in a supra-subduction zone even though they originated from a MOR-like setting. A simplified scenario regarding the early geodynamic evolution of the Philippines is proposed on the basis of the geochemical signatures of the ophiolites, their ages of formation and the ages and origins of the oceanic basins actually bounding the archipelago, including basins presumed to be now totally consumed. This scenario envisages the early development of the archipelago to be largely dominated by the opening and closing of oceanic basins. Fragments of these basins provided the substratum on top of which the Cretaceous to Recent volcanic arcs of the Philippines were emplaced.

Nature and origin of the Triassic volcanism in Albania and Othrys: a key to understanding the Neotethys opening?

2008 ◽  
Vol 179 (4) ◽  
pp. 411-425 ◽  
Author(s):  
Philippe Monjoie ◽  
Henriette Lapierre ◽  
Artan Tashko ◽  
Georges H. Mascle ◽  
Aline Dechamp ◽  
...  
Keyword(s):  
Eastern Mediterranean ◽  
Volcanic Rocks ◽  
Island Arc ◽  
Late Triassic ◽  
Central Mediterranean ◽  
Crustal Component ◽  
Sw Turkey ◽  
Wide Range ◽  
Back Arc ◽  
Island Basalt

AbstractTriassic volcanic rocks, stratigraphically associated with pelagic or reef limestones, are tectonically juxtaposed with Mesozoic ophiolites in the Tethyan realm. From the central (Dinarides, Hellenides) and eastern Mediterranean (Antalya, Troodos, Baër Bassit) to the Semail nappes (Oman), they occur either associated to the tectonic sole of the ophiolitic nappes or as a distinct tectonic pile intercalated between the ophiolites and other underthrust units. In the Dinaro-Hellenic belt, the Pelagonian units represent the lower plate, which is underthrust beneath the ophiolites. Middle to Late Triassic volcanic sequences are interpreted as the eastern flank of the Pelagonian platform and are therefore considered as a distal, deep-water part of the Pelagonian margin.The Triassic volcanics from Albania and Othrys are made up of basaltic pillowed and massive flows, associated locally with dolerites and trachytes. New elemental, Nd and Pb isotopic data allow to recognize four types of volcanic suites: (1) intra-oceanic alkaline and tholeiitic basalts, (2) intra-oceanic arc-tholeiites, (3) back-arc basin basalts, (4) calc-alkaline mafic to felsic rocks. Nd and Pb isotopic initial ratios suggest that the within-plate volcanic rocks were derived from an enriched oceanic island basalt type mantle source, devoid of any continental crustal component. The lower εNd value of the trachyte could be due to assimilation of oceanic altered crust or sediments in a shallow magma chamber. Island arc tholeiites and back-arc basin basalts have a similar wide range of εNd. The absence of Nb negative anomalies in the back-arc basin basalts suggests that the basin floored by these basalts was wide and mature. The high Th contents of the island arc tholeiites suggest that the arc volcanoes were located not far away from the continental margin.Albania and Othrys volcanics contrast with the Late Triassic volcanism from eastern Mediterranean (SW Cyprus, SW Turkey), which displays solely features of oceanic within plate suites. The presence of back-arc basin basalts associated with arc-related volcanics in Central Mediterranean indicates that they were close to a still active subduction during the Upper Triassic, while back-arc basins developed, associated with within-plate volcanism, leading to the NeoTethys opening.

Lithogeochemistry of volcano-plutonic assemblages of the southern Hanson Lake Block and southeastern Glennie Domain, Trans-Hudson Orogen: evidence for a single island arc complex

1999 ◽  
Vol 36 (2) ◽  
pp. 209-225 ◽  
Author(s):  
Ralf O Maxeiner ◽  
Tom II Sibbald ◽  
William L Slimmon ◽  
Larry M Heaman ◽  
Brian R Watters
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Oceanic Island ◽  
Island Arc ◽  
Island Arcs ◽  
Calc Alkaline ◽  
East Central ◽  
South Central ◽  
Volcaniclastic Rocks ◽  
Flin Flon

This paper describes the geology, geochemistry, and age of two amphibolite facies volcano-plutonic assemblages in the southern Hanson Lake Block and southeastern Glennie Domain of the Paleoproterozoic Trans-Hudson Orogen of east-central Saskatchewan. The Hanson Lake assemblage comprises a mixed suite of subaqueous to subaerial dacitic to rhyolitic (ca. 1875 Ma) and intercalated minor mafic volcanic rocks, overlain by greywackes. Similarly with modern oceanic island arcs, the Hanson Lake assemblage shows evolution from primitive arc tholeiites to evolved calc-alkaline arc rocks. It is intruded by younger subvolcanic alkaline porphyries (ca. 1861 Ma), synvolcanic granitic plutons (ca. 1873 Ma), and the younger Hanson Lake Pluton (ca. 1844 Ma). Rocks of the Northern Lights assemblage are stratigraphically equivalent to the lower portion of the Hanson Lake assemblage and comprise tholeiitic arc pillowed mafic flows and felsic to intermediate volcaniclastic rocks and greywackes, which can be traced as far west as Wapawekka Lake in the south-central part of the Glennie Domain. The Hanson Lake volcanic belt, comprising the Northern Lights and Hanson Lake assemblages, shows strong lithological, geochemical, and geochronological similarities to lithotectonic assemblages of the Flin Flon Domain (Amisk Collage), suggesting that all of these areas may have been part of a more or less continuous island arc complex, extending from Snow Lake to Flin Flon, across the Sturgeon-Weir shear zone into the Hanson Lake Block and across the Tabbernor fault zone into the Glennie Domain.

Petrology and geochemistry of the Kamloops Group volcanics, British Columbia

1981 ◽  
Vol 18 (9) ◽  
pp. 1478-1491 ◽  
Author(s):  
Thomas E. Ewing
Keyword(s):  
British Columbia ◽  
Fractional Crystallization ◽  
Middle Eocene ◽  
Volcanic Rocks ◽  
Primary Source ◽  
Low Pressure ◽  
Rock Types ◽  
South Central ◽  
High K ◽  
Petrology And Geochemistry

The Kamloops Group is an alkali-rich calc-alkaline volcanic suite of Early to Middle Eocene age, widespread in south-central British Columbia. Rock types in the suite range from high-K basalt through andesite to rhyolite. The suite is characterized by relatively high K2O, Sr, and Ba, but low Zr, Ti, and Ni concentrations, only moderate Ce enrichment, and little or no Fe enrichment. Initial ratios 87Sr/86Sr are about 0.7040 in the western half, and about 0.7060 in the eastern half of the study area. No difference in chemistry or mineralogy marks this sharp transition. Chemically similar suites include the Absaroka–Gallatin suite in Wyoming and the lower San Juan (Summer Coon) suite in Colorado. The content of K2O at 60% SiO2 increases regularly eastward across southern British Columbia. The chemical data support the subduction-related continental arc origin of the Kamloops Group volcanics.The volcanic rocks consist in the main of augite–pigeonite andesites ranging from 52 to 62% silica, with subordinate quantities of olivine–augite–pigeonite basalt and biotite rhyodacite and rhyolite. The andesites and basalts were derived by a combination of low-pressure fractional crystallization, higher pressure fractional crystallization, and variable parental magmas, whereas low-pressure fractional crystallization of plagioclase, biotite, and apatite from parental basalt and andesite produced the rhyolites. The parental magmas were basalts and basaltic andesites with high K, Sr, and Ba. The primary source of these magmas is inferred to have been an alkali-enriched hydrous peridotite with neither plagioclase nor garnet present in the residuum.

Early Silurian volcanic rocks at Arisaig, Nova Scotia

1979 ◽  
Vol 16 (8) ◽  
pp. 1635-1640 ◽  
Author(s):  
J. Duncan Keppie ◽  
J. Dostal ◽  
M. Zentilli
Keyword(s):  
Nova Scotia ◽  
Lower Devonian ◽  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Island Arc ◽  
Temporal Association ◽  
Continental Plate

Petrochemical data for the Early Llandovery subaerial volcanic rocks at Arisaig, Antigonish Highlands, Nova Scotia, show that they are a bimodal suite of altered rhyolites and basalts of tholeiitic affinity with some alkaline tendencies extruded in a nonorogenic extensional environment within a continental plate. These data do not support the island arc origin frequently inferred for these rocks but are consistent with recent petrological data from the Silurian – Lower Devonian Coastal Volcanic Belt in southern Maine, which may be a strike continuation of the Arisaig volcanics. The close spatial and temporal association of basalts and rhyolites is attributed to anatexis of the crust upon contact with the rising basaltic magma.

A Cambrian island arc in Iapetus: geochronology and geochemistry of the Lake Ambrose volcanic belt, Newfoundland Appalachians

1991 ◽  
Vol 128 (1) ◽  
pp. 1-17 ◽  
Author(s):  
G. R. Dunning ◽  
H. S. Swinden ◽  
B. F. Kean ◽  
D. T. W. Evans ◽  
G. A. Jenner
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Detailed Examination ◽  
Island Arc ◽  
Trace Element Geochemistry ◽  
Geochemical Type ◽  
Element Geochemistry ◽  
South Central ◽  
Iapetus Ocean ◽  
Felsic Volcanic

AbstractThe Lake Ambrose volcanic belt (LAVB) outcrops as a 45 km long northeast-trending belt of mafic and felsic volcanic rocks along the eastern side of the Victoria Lake Group in south-central Newfoundland. It comprises roughly equal proportions of mafic pillow basalt and high silica rhyolite, locally interbedded with epiclastic turbidites. Volcanic rocks have been metamorphosed in the greenschist facies and are extensively carbonatized.U-Pb (zircon) dates from rhyolite at two, widely separated localities give identical ages of 513 ± 2 Ma (Upper Cambrian), and this is interpreted as the eruptive age of the volcanic sequence. Primitive arc and low-K tholeiites can be recognized on the basis of major and trace element geochemistry, ranging from LREE-depleted to LREE-enriched. Geochemical variation between mafic volcanic types is interpreted predominantly to reflect contrasts in source characteristics and degree of partial melting; some variation within each geochemical type attributable to fractional crystallization can be recognized. Detailed examination of some samples indicates that the heavy REE and related elements have locally been mobile, probably as a result of carbonate complexing.The LAVB is the oldest well-dated island arc sequence in Newfoundland, and perhaps in the Appalachian–Caledonian Orogen. Its age requires modification of widely held models for the tectonic history of central Newfoundland. It is older than the oldest known ophiolite, demonstrating that arc volcanism was extant before the generation of the oldest known oceanic crust in this part of Iapetus. It further demonstrates that there was a maximum of approximately 30 Ma between the rift-drift transition which initiated Iapetus, and the initiation of subduction. This suggests that the oceanic sequences preserved in Newfoundland represent a series of arcs and back arc basins marginal to the main Iapetus Ocean, and brings into question whether the Appalachian accreted terranes contain any remnants of normal mid-ocean ridge type Iapetan crust.

Contribution a l'etude du volcanisme de l'arc des Nouvelles-Hebrides; caracterisation de deux series magmatiques de l'ile d'Erromango

1979 ◽  
Vol S7-XXI (5) ◽  
pp. 631-641 ◽  
Author(s):  
G. Marcelot ◽  
C. Lefevre ◽  
P. Maillet ◽  
R. C. Maury
Keyword(s):  
Fractional Crystallization ◽  
Basaltic Magma ◽  
Island Arc ◽  
Calc Alkaline ◽  
Early Stages ◽  
New Hebrides ◽  
Volcanic Series

Abstract The volcanic series of Mt Rantop and Robertson's Thumb, Erromango Island, New Hebrides, formed by fractional crystallization of orogenic basaltic magma of near-island-arc tholeiitic type. Differentiation was controlled mainly by separation of plagioclase, olivine and clinopyroxene. The Mt Rantop series is predominantly tholeiitic (plagioclase at the liquidus, late appearance of magnetite, pigeonite in microphenocrysts, and Fe and Ti remaining constant or increasing in the early stages of differentiation); those of Robertson's Thumb are mostly calc-alkaline (magnetite at the liquidus, late appearance of plagioclase, olivine quickly becoming unstable, orthopyroxene in phenocrysts and early decrease of Fe and Ti). The compositional differences reflect higher fO <sub>2</sub> and PH <sub>2</sub> O in Robertson's Thumb during fractional crystallization.

The Twillingate Granite and nearby Volcanic Groups: An Island Arc Complex In Northeast Newfoundland

1975 ◽  
Vol 12 (6) ◽  
pp. 982-995 ◽  
Author(s):  
Harold Williams ◽  
John G. Payne
Keyword(s):  
Volcanic Rocks ◽  
Metamorphic Grade ◽  
Island Arc ◽  
Granitic Rocks ◽  
Arc Volcanism ◽  
Pillow Lavas ◽  
The North ◽  
Structural Style ◽  
Lower Ordovician ◽  
Island Arc Volcanism

The Twillingate Granite cuts mafic pillow lavas and silicic fragmental volcanic rocks of the Sleepy Cove Group. The granitic rocks are soda-rich and they vary from intensely foliated and mylonitic in the south to mildly foliated and massive toward the north. The Sleepy Cove volcanic rocks show similar structural and metamorphic variations from lineated amphibolitic pillow lavas, to elongated pillows of greenschist metamorphic grade, to slightly metamorphosed and relatively undeformed pillow lavas.The collective terrane occupied by the Twillingate Granite and Sleepy Cove Group is virtually surrounded by intrusive mafic dikes that are integral and coeval parts of the Moretons Harbour and Herring Neck Groups. The dikes decrease in abundance away from the contacts of the collective Twillingate – Sleepy Cove terrane. The essentially intrusive contact is modified by faults and locally, the profuse dike swarms are absent.Regional relationships, thickness, lithofacies, and petrochemistry all indicate that the Moretons Harbour and Herring Neck Groups relate to an episode of Lower Ordovician island arc volcanism. Intrusive relationships and contrasts in structural style and metamorphic grade indicate that the Twillingate Granite and Sleepy Cove Group are older. These older rocks are also interpreted as island arc derivatives, so that in their present position, they may represent the remnant of a partly deformed and metamorphosed older arc that is now bordered by relatively undeformed Lower Ordovician volcanic rocks.Similar relationships within transported sequences of western Newfoundland suggest a central Newfoundland island arc provenance for the transported Little Port Complex.

scholarly journals Petrogeochemical features of the Neogene collision volcanism of the Lesser Caucasus (Azerbaijan)

2020 ◽  
Vol 29 (2) ◽  
pp. 289-303
Author(s):  
Nazim A. Imamverdiyev ◽  
Minakhanym Y. Gasanguliyeva ◽  
Vagif M. Kerimov ◽  
Ulker I. Kerimli
Keyword(s):  
Fractional Crystallization ◽  
Water Pressure ◽  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Fluid Pressure ◽  
Material Balance ◽  
Parental Magma ◽  
High Alumina ◽  
Fractionation Process ◽  
Lesser Caucasus

The article is devoted to the petrogeochemical features of Neogene collision volcanism in the central part of the Lesser Caucasus within Azerbaijan. The main goal of the study is to determine the thermodynamic conditions for the formation of Neogene volcanism in the central part of the Lesser Caucasus using the available petrogeochemical material. Using factor analysis, as well as the “IGPET”, “MINPET”, “Petrolog-3” programs, material balance calculations were performed that simulate the phenocryst fractionation process, the crystallization temperature, pressure, and figurative nature of the rock-forming minerals of the formation rocks were calculated. It was determined that at the early and middle stages of crystallization of the rocks of the andesite-dacite-rhyolite formation, the fractionation of amphibole played an important role in the formation of subsequent differentiates. Based on computer simulation, it was revealed that rocks of the andesite-dacite-rhyolite formation were formed by fractional crystallization of the initial high-alumina basaltic magma of high alkalinity in the intermediate magma foci. The calculations of the balance of the substance, simulating the process of fractionation of phenocrysts, as well as magnetite, confirmed the possibility of obtaining rock compositions from andesites to rhyolites as a result of this process. In this case, the process of crystallization differentiation was accompanied by processes of contamination, hybridism and mixing. Based on the geochemical features of rare and rare-earth elements, changes in their ratios, the nature of the mantle source and the type of fractionation process are determined. It was revealed that the enrichment of formation rocks by light rare earths, as well as by many incoherent elements, is associated with the evolution of enriched mantle material. Under high water pressure, as a result of the fractionation of olivine and pyroxene, high-alumina basalts are formed from primary high-magnesian magma, which can be considered parental magma. It was established that, in contrast to the elevated Transcaucasian zone in the more lowered East Caucasus, under conditions of increased fluid pressure and reduced temperature, the melt underwent fractional crystallization in the intermediate centers, being enriched with alkaline, large-ion lithophilic elements, light REEs, etc. This is evidenced by the presence of large crystals of feldspars, the contamination of these minerals by numerous crystals of biotite, magnetite, several generations of these minerals, zonality, as well as the presence of related “water” inclusions, such as hornblendites, hornblende gabbro, etc. The physicochemical conditions for the formation of Neogene volcanic rocks of the Lesser Caucasus are determined.

scholarly journals Petrology, Geochemical Characteristics, Tectonic Setting, and Implications for Chromite and PGE Mineralization of the Hongshishan Alaskan-Type Complex in the Beishan Orogenic Collage, North West China

2021 ◽  
Vol 9 ◽  
Author(s):  
Zhaolin Wang ◽  
Xiaoming Zheng ◽  
Guixiang Meng ◽  
Hejun Tang ◽  
Tonghui Fang
Keyword(s):  
Fractional Crystallization ◽  
Tectonic Setting ◽  
Orogenic Belt ◽  
Geochemical Characteristics ◽  
North West ◽  
The North ◽  
Light Yellow ◽  
Type Complex ◽  
Mafic Complex ◽  
Beishan Orogenic Collage

The Hongshishan mafic-ultramafic complex is situated in the north of the Beishan orogenic collage and the southern part of the Central Asian Orogenic Belt. This paper outlines the petrological, geochemical, and mineralogical data of the Hongshishan ultramafic–mafic complex in the Beishan orogenic collage to constrain its tectonic setting and mineralization. The lithological units of the complex include dunite, clinopyroxene peridotite, pyroxenite, and gabbro. The complex showed concentric zonation, from clinopyroxene peridotite and dunite in the core to pyroxenite and gabbro in the margin. These ultramafic–mafic rocks are characterized by cumulate and layering textures. Field observations, petrography, and significant elemental composition variation, a decreasing sequence of ferromagnesian minerals (Mg#), olivine Fo, and spinel Cr#, all show fractional crystallization trends from dunites through clinopyroxene peridotite and pyroxenite, to gabbros. There are systematic trends among the primary oxides, e.g., CaO, TiO2, and Al2O3, with MgO, suggesting a fractional crystallization trend. SiO2 and Al2O3 increased, which coupled with decreasing MgO, suggested olivine fractionation. The negative correlations of CaO and Al2O3 with MgO meant the accumulation of spinel and mafic minerals. The compositions of olivines from the dunite and clinopyroxene peridotite in the Hongshishan plot within the Alaskan Global trend fields displayed a typical fractional crystallization trend similar to olivines in an Alaskan-type complex. The clinopyroxenes in the clinopyroxene peridotite primarily occur as a diopside and appear in the field of an Alaskan-type complex. The absence of orthopyroxene, less hydrous, and free of fluid inclusions in the chrome spinels means the absence of a magmatic origin of chromite-bearing peridotites in hydrous parental melts or scarce hydrous melts. Serpentinization, carbonatization, subduction modification, and enrichment may account for the LILE-enrichment and HFSE-depletion of peridotite rocks. Negative Eu anomalies and REE fractionations of mafic-ultramafic rocks may not be directly attributed to crustal assimilation. Petrological, mineralogical, and geochemical characteristics indicated the Hongshishan complex is not the member compositions of a typical ophiolite. However, it displays many similarities to Alaskan-type mafic-ultramafic intrusions related to subduction or arc magmas setting at ∼366.1 Ma and suffered subduction modification and enrichment. The Hongshishan complex is a unique Ir-Ru-rich chromite deposit in the southern margin of the Altaids orogenic belt. Chromites occur primarily in light yellow dunites, with banded, lenticular, veined, thin-bedded, and brecciated textures. Part of the chromite enrichment in IPGE (Os, Ir, Ru) and the chondrite-normalized spider diagram of PGE showed steep right-facing sloped patterns similar to those of the PGE-rich ophiolitic chromites.

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