Pyroxene Peridotite Research Articles

Petrogenesis and Sc mineralization potential of the early Silurian Halaguole Alaskan-type complex in the East Kunlun orogenic belt

The Alaskan-type complexes are known for their unique annular lithologic zoning structure and are products of an island arc environment. The Halaguole mafic–ultramafic intrusions in the Kunlun orogenic belt, as reported in this study, exhibit characteristics of an Alaskan-type complex. The complex is composed of peridotite, pyroxenite, and gabbro, with well-defined annular lithological zoning from the core to the margin. The Halaguole mafic–ultramafic intrusions were formed during the Early Silurian period, specifically ranging from 437 Ma to 440 Ma according to Zircon LA-ICP-MS U-Pb dating results. They are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), revealing positive europium (Eu) anomalies and negative niobium (Nb) anomalies. All of these characteristics indicate that they are products of an island arc environment. Based on analysis of zircon’s εHf(t) value (0.25 to 5.14) and model age (TDM1) (842.19 to 1034.61 Ma), it can be deduced that the intrusions originated from an enriched mantle source. The dominant mineral compositions within the Halaguole intrusions include olivine, clinopyroxene, amphibole, biotite, and spinel. With the absence of orthopyroxene, the compositions of these minerals are similar to Alaskan-type complexes. The contents of olvine, biotite, and spinel suggest that the parental melt of these intrusions is an aqueous basaltic magma with island arc properties. The Halaguole mafic–ultramafic intrusions likely formed through partial melting of the mantle wedge metasomatized by fluids from the subduction zone. This process was accomplished by fractional crystallization of magma. Combining the findings of previous research with the evidence presented in this study, it can be deduced that the closure of the Proto-Tethys Ocean occurred during the Early Silurian. Furthermore, it is evident that the Wanbaogou basaltic plateau in the Southern Kunlun Belt (SKB) underwent bidirectional subduction in a north–south direction. Notably, the samples from pyroxene peridotite and gabbro exhibit a significant concentration of Sc, ranging from 41.5 to 224.5 ppm, exceeding industrial grade levels and indicating excellent potential for Sc mineralization.

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex, North Qaidam: Contraints on the Nature of the Alaskan‐type Continental Arc Root

AbstractOrogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity, slab subduction, and melt or fluid metasomatism. To determine the source of the mantle‐derived parental magma of the peridotite and to investigate the metasomatism that it experienced, we undertook an integrated study of the petrography, whole‐rock major‐ and trace‐element compositions, in situ zircon U–Pb geochronology, and mineral major‐ and trace‐element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam. The Halihatu ultramafic–mafic complex is composed of dunite, pyroxene peridotite, and gabbro, which are characteristic of Alaskan‐type complexes. The dunite yields a weighted mean 206Pb/238U age of 479 ± 5 Ma (MSWD = 0.7), which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma. The peridotites have high Mg# (89.8–91.8) and Cr contents (2419–5190 ppm), low Al2O3 (0.20–1.68 wt.%) and Ni (289–1012 ppm) contents, and high olivine Fo contents (87–91), suggesting a large degree (~15%–22%) of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene. This is consistent with estimates of 15%–22.3% partial melting calculated using the Cr# of spinel crystals and with the low Yb (0.04–0.33 ppm) and Y (0.72–1.29 ppm) contents of clinopyroxene crystals. Whole‐rock trace‐element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements, along with high Al2O3 (2.10–6.47 wt.%) and low TiO2 (0.01–0.21 wt.%) contents of clinopyroxene crystals, suggesting an arc magma cumulate trend. These features, along with the high olivine Fo contents (87–91 ppm), imply that the Halihatu peridotite is an Alaskan‐type crustal cumulates derived from Mg‐rich hydrous basaltic melts. The high estimated fO2 (FMQ +1.97 to FMQ +3.81) further supports the idea that they formed in an arc setting. The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine, quenched boundaries between mafic and felsic melts, and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust. Therefore, we propose a new cumulate‐infiltration model for the petrogenesis of Alaskan‐type ultramafic complexes, which improves our understanding of the nature of Alaskan‐type continental arc root.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Santaishan–Yingjiang Ultramafic Rocks in Western Yunnan, China

The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U–Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186–190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182–183 Ma. The content of MgO in the SSPP is relatively high, but the SiO2, Al2O3, CaO and TiO2 content and ΣREE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong–Nujiang suture zone. The SSPP rocks have relatively high (87Sr/86Sr)i ratios (0.7091–0.7131) and positive Hf(t) values (11.2–13.8), with εNd(t) values varying from −1.1 to 9.4. The YHP has relatively low εHf(t) values (3.5 to 6.9), with the Nd–Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr–Nd and Lu–Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong–Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong–Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed.

The Metallogeny of the Lubei Ni–Cu–Co Sulfide Deposit in Eastern Tianshan, NW China: Insights From Petrology and Sr–Nd–Hf Isotopes

The Lubei Ni–Cu–Co deposit situated in western segment of the Huangshan-Jing’erquan mafic–ultramafic rock belt in eastern Tianshan of the Central Asian Orogenic Belt (CAOB). The estimated reserve is approximately 9.11 million tons of ore resources with average grades of 0.82 wt% Ni, 0.52 wt% Cu, and 0.03 wt% Co. The Lubei intrusion is mainly composed of gabbro (phase I), peridotite (phase II), pyroxene peridotite (phase III), olivine pyroxenite (phase IV), and diorite (phase V), which intruded into the early Carboniferous tuffaceous clastic rocks. Zircon Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA–ICP–MS) U–Pb age of the diorite (phase V) from the edge of the intrusion is interpreted as the top-limit metallogenic age, which is consistent with the formation ages of the Huangshan and Xiangshan Ni–Cu deposits in eastern Tianshan. The roughly parallel rare earth element (REE) curves of the Lubei intrusion indicate the magma originated from a homologous source. The slightly enriched large ion lithophile elements (LILE) are compared to high field strength elements (HFSE) with negative Nb and Ta anomalies show that the Lubei intrusion has arc-affiliate geochemical characteristics. The Sr–Nd–Hf isotopes show that the magma was derived from depleted lithospheric mantle, while suffering 4–10% lower crustal contamination with slight contamination of the upper crust. Based on a comprehensive conservation of regional geological, geochemical, and geochronological evidence, the primary magma of the Lubei intrusion was identified that it was derived from the partial melting of metasomatized lithospheric mantle previously modified by subduction events. The Lubei nickel–copper–cobalt sulfide deposit was formed after the primary magma experienced fractional crystallization, crustal contamination, and sulfide segregation in a post-collisional extensional geodynamic setting after the closure of the Kanggur ocean basin in the early Permian.

Mafic-ultramafic magma activity and copper-nickel sulfide metallogeny during Paleozoic in the Eastern Kunlun Orogenic Belt, Qinghai Province, China

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB) has a complex geological structure and diverse magmatic activities, which are closely related to the Qaidam Basin and the Tethys tectonic evolution. There are at least 3 stages mafic-ultramafic rocks occurred in the Early Paleozoic in EKOB. The first stage is the Later-Silurian to Early Devonian, represented by the giant Xiarihamu super large magmatic Cu-Ni deposit, containing about 1.18 million metric tons (Mt) of nickel with average grades of 0.65% Ni, and its age of ore-forming pyroxene peridotite is 411 Ma; The second stage is the Early Carboniferous, represented by the large Shitoukengde magmatic Cu-Ni sulfide deposit, and its ore-forming age of the olivine websterite is 334 Ma; The third stage of mafic-ultramafic rocks occurred mainly during the Middle-Late Triassic, represented by Xiaojianshan, Lalinggaoli, and Kaimuqi complexes, and no economical ore bodies have been found in this period. The authors summarized the difference between the ore-bearing and the nonmineralized mafic-ultramafic rocks in the EKOB. The olivine of the ore-bearing complexes contains higher MgO and SiO2 content but lower FeO and CaO contents, and the clinopyroxene of ore-bearing complexes contains lower FeO and CaO contents. Crustal sulfur contamination is key to the formation of the giant Xiarihamu Ni deposit, and crustal sulfur contamination degree of the giant magmatic Ni deposit is higher than that of large Ni deposit. The above indicators could guide the exploration and evaluation of similar deposits in the EKOB.

Mineralogy, geochemistry and emplacement of the Conakry Igneous Complex, Guinea: implications for the Ni–Cu–PGE mineralization

ABSTRACTThe Conakry Igneous Complex is a mafic-ultramafic intrusion emplaced contemporaneously with the opening of the Atlantic, forming a complex, 55 km x 5 km dyke-like body within which three main episodes of injection have been recognized, characterized by a lack of mineral layering. Unit 1 consists of dunite and related facies, Unit 2 of wehrlite and pyroxene peridotite and Unit 3 corresponds to various gabbro facies. Units 1 and 2 constitute the Kaloum Peninsula; Unit 3 is its NW extension, forming the 1010 m high Mount Kakoulima. Unit 3 intrudes the two previous units and corresponds to a tholeiitic liquid that crystallized in an almost closed system, and thus exhibits a strong differentiation trend, in contrast to Units 1 and 2. Mineral compositions suggest the existence of a deeper magma chamber where a first stage of differentiation occurred.Disseminated base-metal sulfides (BMS) are present in all units of the complex and earlier descriptions have mentioned a “massive sulfide layer” with 2 to 4 g/t PGE. Platinum-group minerals (PGM) are almost everywhere included in or attached to composite Ni–Fe–Cu sulfides. Most PGM grains form complex associations resulting either from exsolution or alteration. It is characteristic of the Conakry Igneous Complex PGM, described here for the first time, to be dominated by (Pd,Pt)(Te,Bi) minerals with rare Pd,Sn and Pd,Pb compositions and an absence of Pt,Pd sulfides and Pt,Pd antimonides.The constant association of the PGM with the BMS shows that the magmatic sulfide liquid acts as an efficient collector of PGE. In such a dynamic environment, the process leading to the formation of massive sulfides must be sought in the accumulation of sulfides in the conduit following host-rock assimilation. Accordingly, considering the multiple injection processes that characterize the whole intrusion, the potential for discovering additional Ni–Cu–PGE mineralization in the Conakry Igneous Complex remains high.

Tectonic significance of the Late Carboniferous Zhunmubutai ophiolitic mélange from Xi‐Ujimqin, Inner Mongolia

The Zhunmubutai ophiolitic mélange is a newly discovered ophiolite suite located between the Solonker‐Linxi ophiolite belt and the Erenhot‐Hegenshan‐Xi‐Ujimqin ophiolite belt, the central part of Xilinhot Microcontinent (XLMC). It was composed of plagiogranite, pyroxene peridotite, pillow basaltic andesite, and associated mafic rocks and surrounded by a quartz diorite pluton and mélange zone containing sandstones and slates of Early Permian Shoushangou Formation. In this study, zircon grains from a quartz diorite and a plagiogranite yield 206Pb/238U ages of 313 ± 1 Ma (MSWD = 1.60, n = 20) and 328 ± 1 Ma (MSWD = 0.9, n = 93), respectively. Sixty‐four zircons from the slate yielded ages ranging from 307 to 2,313 Ma, and with a peak at 340 Ma. These data indicate the Zhunmubutai ophiolitic mélange formed 300 to 340 Ma. The Zhunmubutai plagiogranites that have low contents of K2O (0.07–0.33 wt.%), K2O/Na2O (0.02–0.09), and Rb/Sr (0.01–0.05) show an oceanic plagiogranites affinity. They are also characterized by low contents of TiO2, Nd, Ta, and significant LREE enrichment. Furthermore, the εHf (328 Ma) values range from 14.23 to 18.87. These features suggest that the plagiogranite was originated by the anatexis of amphibolites and with crustal contamination. Combined with previous studies, suggest that the Zhunmubutai ophiolitic mélange may have formed in an extensional environment during the early stages of opening of an ocean basin.

Geochronology, Geochemistry and Sr‐Nd‐Pb‐Hf Isotopes of No. I Complex from the Shitoukengde Ni–Cu Sulfide Deposit in the Eastern Kunlun Orogen, Western China: Implications for the Magmatic Source, Geodynamic Setting and Genesis

AbstractThe Shitoukengde Ni‐Cu deposit, located in the Eastern Kunlun Orogen, comprises three mafic–ultramafic complexes, with the No. I complex hosting six Ni‐Cu orebodies found recently. The deposit is hosted in the small ultramafic bodies intruding Proterozoic metamorphic rocks. Complexes at Shitoukengde contain all kinds of mafic‐ultramafic rocks, and olivine websterite and pyroxene peridotite are the most important Ni‐Cu‐hosted rocks. Zircon U‐Pb dating suggests that the Shitoukengde Ni‐Cu deposit formed in late Silurian (426–422 Ma), and their zircons have εHf(t) values of −9.4 to 5.9 with the older TDM1 ages (0.80–1.42 Ga). Mafic‐ultramafic rocks from the No. I complex show the similar rare earth and trace element patterns, which are enriched in light rare earth elements and large ion lithophile elements (e.g., K, Rb, Th) and depleted in heavy rare earth elements and high field strength elements (e.g., Ta, Nb, Zr, Ti). Sulfides from the deposit have the slightly higher δ34S values of 1.9–4.3± than the mantle (0 ± 2±). The major and trace element characteristics, and Sr‐Nd‐Pb and Hf, S isotopes indicate that their parental magmas originated from a metasomatised, asthenospheric mantle source which had previously been modified by subduction‐related fluids, and experienced significant crustal contamination both in the magma chamber and during ascent triggering S oversaturation by addition of S and Si, that resulted in the deposition and enrichment of sulfides. Combined with the tectonic evolution, we suggest that the Shitoukengde Ni‐Cu deposit formed in the post‐collisional, extensional regime related to the subducted oceanic slab break‐off after the Wanbaogou oceanic basalt plateau collaged northward to the Qaidam Block in late Silurian.

Alaskan-type Kedanshan intrusion (central Inner Mongolia, China): Superimposed subduction between the Mongol-Okhotsk and Paleo-Pacific oceans in the Jurassic

The Xing’an-Inner Mongolia accretionary belt in the eastern Central Asian Orogenic Belt (CAOB) was produced by the subduction of three oceanic plates: the Paleo-Asian, Mongol-Okhotsk and Paleo-Pacific oceanic plates. The interactions between these plates remain unclear. Here we report an Alaskan-type ultramafic-mafic intrusion in the Kedanshan area, central Inner Mongolia, China. The main lithologies of this intrusion include cumulate dunite, pyroxene peridotite, olivine pyroxenite and cumulate gabbro, with late gabbroic/anorthositic dykes. Minerals and whole-rock compositional variations display characteristics of an arc cumulate trend (Alaskan-type), through fractional crystallization of Mg-rich and hydrous basaltic magma associated with oceanic subduction. Zircons from two gabbro samples yield Early Jurassic ages of 193 ± 6 Ma and 179 ± 4 Ma, respectively. We conclude that this ultramafic-mafic complex is an accumulated intrusion from an arc-related, high-Mg magma chamber above a supra-subduction zone. Considering the ages, location and tectonic setting of the complex, we suggest that it was most likely generated by melting of a large and triangle-shaped mantle wedge during superimposed subduction between the Mongol-Okhotsk and the Paleo-Pacific oceanic plates in the Jurassic.

Zircon U–Pb age, Hf isotope and geochemistry of Carboniferous intrusions from the Langshan area, Inner Mongolia: Petrogenesis and tectonic implications

Late Paleozoic was a critical period for the tectonic evolution of the northern margin of the Alxa-North China craton, but the evolutionary history is not well constrained. The Carboniferous intrusions in the Langshan area in the western part of the northern margin of the Alxa-North China craton are mainly composed of tonalite, quartz diorite, olivine gabbro and pyroxene peridotite. Zircon LA-ICP-MS U–Pb dating indicates that the Langshan Carboniferous intrusions were emplaced at ca. 338–324Ma. The quartz diorites are characterized by high amounts of compatible trace elements (Cr, Ni and V) and high Mg# values, which may suggest a significant mantle source. The positive Pb and negative Nb–Ta–Ti anomalies, the variable εHf(t) (−6.9 to 2.0) values and the old Hf model ages (1218–1783Ma) imply some involvement of ancient continental materials in its petrogenesis. The tonalite has relatively high Sr/Y ratios, low Mg#, Yb and Y contents, features of adakite-like rocks, negative εHf(t) values (−9.8 to −0.1) and older Hf model ages (1344–1953Ma), which suggest significant involvement of ancient crust materials and mantle-derived basaltic component in its petrogenesis. The high Mg# values, high Cr and Ni contents, and low Zr and Hf contents of the mafic–ultramafic rocks show evidence of a mantle source, and the relatively low zircon εHf(t) values (−5.9 to 3.2) might point to an enriched mantle. The trace element characteristics indicate the influence of subducted sediments and slab-derived fluids. In the tectonic discrimination diagrams, all the rocks plot in subduction-related environment, such as volcanic arc and continental arc. Considering the regional geology, we suggest that the Carboniferous intrusions in the Langshan area were likely emplaced during the late stage of the southward subduction of the Paleo-Asian Ocean plate, which formed a continental arc along the northern margin of the Alxa-North China craton.

Geology, petrology and geochemistry of the Baishiquan Ni–Cu-bearing mafic–ultramafic intrusions in Xinjiang, NW China: Implications for tectonics and genesis of ores

The Baishiquan mafic–ultramafic intrusions associated with magmatic Ni–Cu–(PGE) sulfide deposit are located in the northern margin of the Central Tianshan Block in northern Xinjiang, NW China. The intrusions consist of olivine pyroxenite, pyroxene peridotite, troctolite, hornblendite, gabbro and diorite. The peridotite and pyroxenite are the main host rock for the Cu–Ni ores. The crystallization sequence of the intrusions is generally as follows: olivine – plagioclase – orthopyroxene – clinopyroxene – amphibole – biotite. The chemical compositions of the rocks have SiO 2 (38–51%), MgO (7–32%) and Al 2O 3(4.2–18%), and relatively low TiO 2(0.32–0.96%) and K 2O+Na 2O (0.06–3.4%). In general, they are characterized by enrichment in large ion lithophile elements (LILE, e.g., Rb, Sr, K, U, Pb and Th) and light rare earth elements (LREE), depletion in high field strength elements (HFSE, e.g., Nb, Ta, Ti and P) relative to primitive mantle and MORB. The absolute PGE abundances are low. Sr–Nd–Pb isotope data suggest a mixture of multi-components of mantle sources, including depleted asthenospheric mantle end-member with MORB-like isotopic signature and EMIIend-member related to subducted oceanic crust. These geochemical characteristics indicate that the parental magma was high-magnesium tholeiitic. Sulfur had reached saturation and immiscible sulfides droplets segregated from silicate magmas before their emplacement. Contamination by a small amount of subducted-related components combined with crystal fractionation resulted in the Ni–Cu-(PGE) mineralization.

Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development

The intrusive rocks of the Nipigon Embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the ~1.1 Ga Mesoproterozoic Mid-continent Rift. New geochemical data indicate that the sills can be subdivided into five distinct groups: three mafic sills (Nipigon, Inspiration, and McIntyre diabase sills), with the Nipigon sills forming the bulk of the outcrop, and two spatially restricted ultramafic to mafic sills (Jackfish and Shillabeer sills). The latter mafic sills are typically massive, medium-grained, intergranular textured gabbros ranging in thickness from a few metres to more than 250 m. Two of the ultramafic intrusions included in this study (Disraeli and Hele) consist of a core of pyroxene peridotite with olivine gabbro along the margins. The geochemical characteristics of the ultramafic intrusions and diabase sills are consistent with plume-derived melts that have undergone subsequent fractionation and been contaminated by continental crust, likely at depth, but a few samples from the Hele and Disraeli intrusions have the characteristics of primary, uncontaminated melts that have been rapidly transported through the lithosphere with little interaction with wall rocks. The field and geochemical characteristics of the intrusions and sills are consistent with the ultramafic intrusions having been emplaced before the diabase sills and indicate that the history of the Midcontinent Rift is more complex and protracted than previously recognized.

Electrical Conductivity of Granite, Basalt and Pyroxene Peridotite at High Temperature and Pressure

AbstractExperiment results obtained for the conductivity of granite, basalt and pyroxene peridotite at high temperature (563 − 1173K) and pressure (1.0 − 0.5GPa) indicate that electrical conductivity increases with increasing temperature. The increase in conductivity can be 105 times and may be the result of rock partial‐melting.

Geology and genesis of the mafic-ultramafic complexes in the Huangshan-Jingerquan (HJ) belt, East Xinjiang

More than twenty mafic-ultramafic complexes, which host several mediumor large-sized Cu−Ni deposits, occur along the Huangshan-Jingerquan (HJ) belt in East Xinjiang. Rock types in these complexes are predominated by peridotite, pyroxene peridotite, olivine pyroxenite, gabbronorite, orthopyroxene gabbro, troctolite, gabbro and diorite. The ultramafic rocks are relatively Fe-enriched and are characterized by an assemblage of olivine+orthopyroxene+clinopyroxene+hornblende±plagioclase without obvious metamorphic textures. Chemically, these complexes are relatively Fe-enriched and show a tholeiitic trend of evolution. The complexes in this belt are intruded under the extensional environment in a Mid-Carboniferous back-arc basin. They can be considered as a new type of mafic-ultramafic complexes in orogenic belts, as designated by the name of the East-Xinjiang-type complexes.

Mineralogy of peridotitic compositions under upper mantle conditions

An experimental study of the stability fields at high pressure of plagioclase peridotite, aluminous pyroxene (±spinel) peridotite and garnet peridotite, has been carried out in compositions matching estimates of the average undifferentiated upper mantle (pyrolite). The appearance of garnet at high pressures results from either of two complex reactions: 1. a.|spinel+pyroxene ⇌ olivine+garnet, (solid solutions) 2. b.|aluminous pyroxene ⇌ garnet+pyroxene (lower Al 2O 3). (solid solution) In the model pyrolite composition, garnet appears at 21 kb (1100 °C) to 24 kb (1300 °C) by reaction (a) but at temperatures above 1300 °C spinel is absent from the low pressure assemblage and garnet develops by reaction (b) at pressures of 24 kb (1300 °C) to 31 kb (1500 °C). Reactions (a) and (b) have very different P, T gradients. In the garnet peridotite field, the amount of garnet is inversely proportional to the alumina content of the pyroxenes. This is confirmed by microprobe analyses of orthopyroxene (ranging from 6.0 to 2.2% Al 2O 3, 0.7–0.9% Cr 2O 3) coexisting with garnet, olivine and clinopyroxene. Coexisting clinopyroxenes have higher R 2O 3 content and exhibit a variable, temperature dependent enstatite solid solution. The experimental data are applied to estimate mineralogical, density and seismic velocity variations along oceanic and continental geothermal gradients in a pyrolite upper mantle. Although the mineralogical variation may produce a low velocity zone, the alternate explanation of incipient or minor partial melting is preferred. Experimental data on the maximum stability limit of amphibole in pyrolite (with 0.1–0.2% water) are presented showing that amphibole breaks down to olivine+pyroxenes+garnet+fluid phase at approximately 29 kb, 1000 °C. The instability of amphibole at depths >80–90 km is considered to result in a zone of incipient or minor melting in a pyrolite upper mantle with 0.1–0.2% water. The nature of the liquid in this zone is considered to be highly undersaturated olivine basanite or olivine nephelinite.

Chemical composition of the upper mantle

Extraterrestrial data are not likely to provide quantitative values for the chemical composition of the upper mantle. Three groups of terrestrial materials, ultramafic rocks, olivine nodules from basalts, and garnet peridotite nodules from kimberlites, probably are directly derived from the upper mantle. Each group has a wide range of composition. However, the estimates for the composition of original or undepleted mantle derived independently from each source approach a common value. When this composition is recalculated as probable mineral assemblages, the pyroxene peridotite and garnet peridotite zones both contain about 65% of olivine. The range of composition of each group is consistent with compositional heterogeneity in the upper mantle.

The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structure

An experimental study of the stability fields at high pressure of garnet peridotite and aluminous pyroxene peridotite has been carried out in compositions matching estimates of the average, undifferentiated upper mantle (pyrolite). The appearance of garnet at higher pressures in the pyrolite compositions results from either of two reactions: (1) spinel + orthopyroxene ⇋ olivine + garnet (2) aluminous pyroxene ⇋ garnet + pyroxene (lower alumina). The role of spinel in the lower pressure assemblages is sensitively dependent on temperature and bulk composition. For the pyrolite composition preferred for the upper mantle, spinel is absent above 1300°C and the first appearance of garmet at pressures of 24 kb (1300°C) to 31 kb (1500°C) is due to reaction (2). In this composition garnet does not appear on the pyrolite solidus nor in its melting interval at pressures below 31.5 kb.At temperatures less than 1300°C, garnet appears at 21 kb (1100°C) to 24 kb (1300°C) and develops by reaction (1) at the expense of spinel. The amount of garnet formed by this reaction is dependent on the alumina content of the pyroxenes and throughout the temperature range 1100–1500°C the amount of garnet present increases markedly this is matched by decreasing Al2O3 content of the pyroxenes and allow preliminary estimation of P, T-dependent curves of constant Al2O3 content for orthopyroxene in garnet peridotite assemblages.The experimental data are applied to estimate density and seismic velocity variations along oceanic and continental geothermal gradients in a pyrolite upper mantle. It is emphasized that seismic velocity distributions are sensitively affected by variations in geothermal gradient and by mantle chemical composition, e.g. by variation from pyrolite to residual, refractory, dunite-peridotite. It is suggested that seismic velocity (Vs) variation in an oceanic upper mantle of pyrolite composition may be characterized by two low-velocity channels: (1) a narrow, but sharply defined low-velocity zone at 60–70 km depth caused by mineralogical zoning in the upper mantle; (2) a broader low velocity zone at 120–150 km depth defined primarily by the critical gradient for Vs in the upper mantle but accentuated by mineralogical variations in pyrolite.

A preliminary note on the syenitic rocks form the Osawa-gawa area near Matsumae-mati, Oshima Province, southwestern Hokkaido

The Matsumae district is situated in the southern parts of the Oshima Peninsula in Hokkaido. Acidic and basic igneous rocks occur in the present area as a small mass at some places, that is, Osawa-gawa, Araya-gawa and Matsukura-gawa. The rocks consist mainly of pyroxene-biotite-hornblende diorite, hornblende-biotite granite, hornblende gabbro and pyroxene peridotite. The related minor rocks such as pegmatite, aplite and fine-grained granite are found in the dioritic complex. The writer describes the petrographical characters of the syenodiorite and melanocratic hornblende syenite found in the dioritic mass of the Osawa-gawa, and gives some considerations on their genesis.

The Origin of New Zealand Ultramafic Intrusions

Dun Mountain, Red Hills, and Red Mountain, are three of the largest ultramafic bodies associated with Permian rocks in the South Island of New Zealand. In the three intrusions a central core of relatively unserpentinized dunite, harzburgite, and pyroxene peridotite, is surrounded by a margin of serpentinite. At the western margin of the Red Hills intrusion, the Permian volcanics show high-grade thermal metamorphism with development of pyroxene hornfels at the contact. Layering between dunite and harzburgite is well developed at Red Hills, and on a larger scale at Dun Mountain. Layers at Red Hills are ¼ inch to 20 inch thick and can be traced over an entire outcrop. Harzburgites show poikilitic texture and euhedral crystals. Dimensional orientation of platy olivines, which lie on the broad (010) face in the plane of layering, produces a strong fissility in some of the rocks. Petrofabric analysis shows a strong X maximum normal to the plane of layering and a Z maximum in the plane of layering. Size analysis of crystals demonstrates a size grading in the enstatites from large at the bottom of a layer to small at the top. Olivines are also size-graded, but increased secondary enlargement of crystals at the top of a layer has complicated this. Olivine (Fo93·8—89·4) is the main mineral in all the rocks. Enstatite (En93—88·5) contains numerous exsolution lamellae of clinopyroxene on (100). Chrome diopside is nearly always present in small amounts and picotite is a constant accessory. Comparison of the textures and mineral chemistry of the ultramafic rocks with those of the ultramafic parts of layered intrusions, suggests that the New Zealand rocks have been derived by gravitational differentiation from a tholeiitic magma. It is proposed that the intrusions represent the sub-volcanic differentiates of a chain of Permian volcanoes on the margin of the New Zealand geosyncline, and that the associated olivine-poor tholeiitic Permian basalts are the extrusive differentiates.

Crustal structure from pacific basin to central Nevada

Seismic refraction, gravity, and phase velocity data are generally consistent with an interpretation of crust and upper mantle structure from the Pacific basin across the edge of the continent, the Coast Ranges, the Great Valley of California, the Sierra Nevada, and part of the Basin and Range province. The derived structure consists of a thin oceanic crust beneath the Pacific basin, thickening at the continental slope to a little more than 20 km under the Coast Ranges and Great Valley, and thickening further under the Sierra Nevada and Basin and Range province. The ocean basin is underlain by a normal high-velocity mantle, but the entire continental area is underlain by an anomalous upper mantle whose velocity and density are about 3% less than normal. The anomalous mantle extends to a depth of about 50 km and its lower boundary may be gradational. The crust under the Sierran highland region (Sierra Nevada and western Basin Ranges) is thicker than that in the area to the east; this ‘root’ of the highland is not limited to the Sierra Nevada proper. The root and the voluminous plutonic rocks constitute the core of the Cordilleran eugeosyncline. The root derived from a simple gravity model is about 32 km deep and, from seismic refraction data, more than 40 km deep. To reconcile the two a denser root is required, and it is suggested that this represents a residue of partial melting to form the plutonic rocks. The anomalous upper mantle, which is probably composed of plagioclase peridotite derived by a phase change from garnet peridotite or pyroxene peridotite, can explain no more than about 1 km of the Cenozoic uplift in the region. To explain greater uplift by this means requires renewal of the anomalous mantle, possibly by differentiation into basalt and peridotite followed by convective overturn to bring fresh garnet peridotite to the upper mantle. An anomalous upper mantle characterizes many regions of recent tectonic activity. Superimposed upon the regional gravity anomalies are anomalies caused by large anomalous masses in the upper part of the crust: (1) a negative anomaly of about 50 mgal caused by sedimentary rocks of the Great Valley, (2) positive anomalies associated with the ‘greenstone belt’ beneath the Great Valley and the western Sierra Nevada, (3) negative anomalies due to granitic batholiths, and (4) a negative anomaly suggesting thick sedimentary rocks on the continental slope. The positive gravity and magnetic anomaly of the greenstone belt is comparable with anomalies marking the mafic belts of other geosynclines.