Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

<p>Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.</p>

P-T-fluid conditions of mineral equilibria in garnet-biotite crustal xenoliths from the Yubileinaya and Sytykanskaya kimberlite pipes, Yakutian kimberlite province.

&lt;p&gt;This study provides the results of research of the garnet-biotite crustal xenoliths from the Yubileinaya (372&amp;#177;4.8 Ma) and Sytykanskaya (363&amp;#177;13 Ma) kimberlite pipes of the Alakit-Markhinsky field (Siberian craton). Isotopic evidence on zircons from similar crustal xenoliths (Grt+Bt+Pl+Kfs+Qtz&amp;#177;Scp) showed Archean Hf model ages (TDM = 3.13-2.5 Ga) and thus indicated that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time (Shatsky et al., 2016).&lt;/p&gt;&lt;p&gt;The xenoliths are represented by the assemblage Grt+Bt+Pl+Kfs&amp;#177;Opx. Quartz is present only as rare inclusions in garnets. The rocks are coarse-grained, slightly foliated with garnets porphyroblasts of up to 5 cm in size. A spectacular feature of the rocks is an abundance of K-feldspar. Garnet grains are almost compositionally homogeneous, although they show a rimward decrease of the Mg and Ca contents indicating exchange reactions during cooling. Biotites are characterized by high F increasing from 1.5 wt.% in cores up to 2.2 wt.% in rims, as well as TiO&lt;sub&gt;2&lt;/sub&gt; up to 7.8 wt.%, which is typical for high-grade rocks. Orthopyroxene (up to 5.5 wt. % Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) relics are preserved both as inclusions in garnet and as individual grains in the rock matrix. Plagioclase occurs both as separate grains and as lamellae in potassium feldspar.&lt;/p&gt;&lt;p&gt;The bulk chemical compositions correspond to a metagraywacke. The REE spectra in these rocks are rather flat with slight enrichment in LREE. All the studied rocks are characterized by a distinct negative Eu anomaly (Eu/Eu* = 0.31-0.45).&lt;/p&gt;&lt;p&gt;Calculations using the PERPLEX software version 6.7.6 (Connolly, 2005) for Mg and Ca in Grt, Mg in Bt, and Ca in Pl indicated temperatures 630-730&amp;#176;C and pressures 5.8-7.2 kbar for the rocks. However, equilibria involving Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; in orthopyroxene corresponds to temperatures of 750-800&lt;sup&gt;o&lt;/sup&gt;&amp;#1057; at a similar pressure. It indicates that metamorphism of the garnet-biotite rocks reached higher temperatures, but they were actively modified later during cooling and insignificant decompression (by about 1 kbar). Calculations using the TWQ software version 2.3 (Berman, 2007) indicate consistent temperatures 610-680&amp;#176;C for the garnet-orthopyroxene and 640-690&lt;sup&gt;o&lt;/sup&gt;C for garnet-biotite Mg-Fe exchange equilibria. Calculations using the Grs+2Prp+Kfs+H&lt;sub&gt;2&lt;/sub&gt;O=Phl+3En+3An equilibrium demonstrated water activity below 0.1. Such low water activity could indicate an influence of highly concentrated alkaline Cl-F-bearing brines. This assumption is confirmed by extensive development of potassium feldspar, absence of quartz in the matrix, and elevated Cl contents of biotite, 0.1-0.3 wt. % at high #Mg (&gt;0.7) and F content.&lt;/p&gt;&lt;p&gt;&lt;em&gt;The study is supported by the Russian Science Foundation project 18-17-00206.&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;strong&gt;References &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Berman, R. G. (2007). winTWQ (version 2.3): a software package for performing internally-consistent thermobarometric calculations.&amp;#160;Geological survey of Canada, open file,&amp;#160;5462, 41.&lt;/p&gt;&lt;p&gt;Connolly, T. M., &amp; Begg, C. E. (2005).&amp;#160;Database systems: a practical approach to design, implementation, and management. Pearson Education.&lt;/p&gt;&lt;p&gt;Shatsky, V. S., Malkovets, V. G., Belousova, E. A., ... &amp; O&amp;#8217;Reilly, S. Y. (2016). Tectonothermal evolution of the continental crust beneath the Yakutian diamondiferous province (Siberian craton): U&amp;#8211;Pb and Hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes.&amp;#160;Precambrian Research,&amp;#160;282, 1-20.&lt;/p&gt;

Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaiá-II kimberlite, Alto Paranaíba Province, southeast Brazil

ABSTRACT The Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12–2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.

The Medicine Hat Block and the Early Paleoproterozoic Assembly of Western Laurentia

The accretion of the Wyoming, Hearne, and Superior Provinces to form the Archean core of western Laurentia occurred rapidly in the Paleoproterozoic. Missing from Hoffman’s (1988) original rapid aggregation model was the Medicine Hat block (MHB). The MHB is a structurally distinct, complex block of Precambrian crystalline crust located between the Archean Wyoming Craton and the Archean Hearne Province and overlain by an extensive Phanerozoic cover. It is distinguished on the basis of geophysical evidence and limited geochemical data from crustal xenoliths and drill core. New U-Pb ages and Lu-Hf data from zircons reveal protolith crystallization ages from 2.50 to 3.28 Ga, magmatism/metamorphism at 1.76 to 1.81 Ga, and εHfT values from −23.3 to 8.5 in the Archean and Proterozoic rocks of the MHB. These data suggest that the MHB played a pivotal role in the complex assembly of western Laurentia in the Paleoproterozoic as a conjugate or extension to the Montana Metasedimentary Terrane (MMT) of the northwestern Wyoming Province. This MMT–MHB connection likely existed in the Mesoarchean, but it was broken sometime during the earliest Paleoproterozoic with the formation and closure of a small ocean basin. Closure of the ocean led to formation of the Little Belt arc along the southern margin of the MHB beginning at approximately 1.9 Ga. The MHB and MMT re-joined at this time as they amalgamated into the supercontinent Laurentia during the Great Falls orogeny (1.7–1.9 Ga), which formed the Great Falls tectonic zone (GFTZ). The GFTZ developed in the same timeframe as the better-known Trans-Hudson orogen to the east that marks the merger of the Wyoming, Hearne, and Superior Provinces, which along with the MHB, formed the Archean core of western Laurentia.

Widespread crustal melting since 28 Ma creates the modern flat Tibet

&lt;p&gt;As the largest and highest plateau on Earth, the Tibetan Plateau is distinguished from most other ranges and liner continental orogenic belts (e.g., the Alps) by its broad and flat topography. According to influential numerical and theoretical models, the (former) existence of ductile and molten mid-to-lower crust was an essential contributor to the topographic smoothing process. However, the question of whether the Tibetan Plateau has undergone widespread crustal melting remains highly controversial and hard to prove due to the scarcity of direct evidence from the deep crust. Here we first report on a series of hydrous crustal xenoliths entrained in 28 Ma host lavas from central and northern Tibet. Our new results document the former existence of hydrous crust at 28 Ma as a potentially highly fertile magma source. Quantitative modeling reveals a thermal gradient reaching about 680 &amp;#8451; to 790 &amp;#8451; at a depth of 14 to 40 kilometers, which is significantly lower than that of recent (since 2.3 Ma) evidence for hot Tibetan crust. Petrological data suggest that the initial crustal melting beneath Tibet began at 28 Ma at depths of 23&amp;#8211;40 km (and even deeper) with 0.5&amp;#8211;9.6 vol. % melts, which would lead to a significant reduction of seismic speeds similar to the low-velocity zones observed in the present Tibetan mid-to-lower crust. As the geothermal gradient continued to rise from 28 to 2.3 Ma, wholesale crustal melting (&gt; 20&amp;#8211;30 vol. %) of the mid-to-lower crust beneath Tibet was inevitable and created the modern flat Tibetan Plateau.&lt;/p&gt;