Host Gabbro Research Articles

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Uranium-Lead Systematics of Lunar Basaltic Meteorite Northwest Africa 2977.

Northwest Africa (NWA) 2977 is a lunar basaltic meteorite that was found in 2005 and has been classified as an olivine cumulate gabbro. This meteorite contains a shock melt vein (SMV) induced by an intense shock event. We report herein on an in-situ analysis of phosphates in the host gabbro and the shock vein for the U-Pb dating of NWA 2977 using an ion microprobe, NanoSIMS. The majority of the analyzed phosphates, in both the SMV and host-rock, lie on a linear regression in 238U/206Pb-207Pb/206Pb-204Pb/206Pb three-dimensional space, indicating a total Pb/U isochron age of 3.15±0.12 Ga (95% confidence level), which is consistent ages determined in previous isotopic studies of NWA 2977 (Sm-Nd age of 3.10±0.05 Ga, Rb-Sr age of 3.29±0.11 Ga, and Pb-Pb baddeleyite age of 3.12±0.01 Ga), and identical to the age of the U-Pb phosphate in a paired meteorite NWA 773, 3.09±0.20 Ga, derived from our dataset. There was no clear difference in the formation age between the phosphates found in the SMV and host-rock, although the shape and size of the grains and the Raman spectra show the evidence of intense shock metamorphism. Based on these findings, the cooling rate of the phosphate was very rapid, constrained to be larger than 140 K/s.

Interplay between crystal-plasticity, fracturing and dissolution-precipitation creep in lower-crustal ultramylonite from hole U1473A, Atlantis Bank, Southwest Indian Ridge

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust.

Petrogenesis of zoned and unzoned mafic pegmatites: An insight from the Palaeoproterozoic mafic-ultramafic Hamn intrusion, Northern Norway

Mafic pegmatites have been reported from various geological environments, including ophiolites, layered magmatic intrusions, and volcanic arcs, but their petrogenesis stayed poorly constrained. This study brings new mineralogical and geochemical data obtained from unzoned and zoned gabbroic pegmatites hosted by the 1.8 Ga mafic-ultramafic Hamn intrusion, Northern Norway, with an aim to improve our understanding of the primary magmatic physicochemical factors that control the development of pegmatitic textures in mafic rocks. The unzoned mafic pegmatites in the Hamn intrusion are hosted in gabbronorite and gabbro. The pegmatites differ macroscopically from the gabbronorite only by grain size and the color of feldspars. The bulk chemical compositions, including REE concentrations and rock-forming mineral assemblages of both rocks, are identical. The mineral composition of the pegmatite and host gabbro overlap. However, the pegmatite experienced epidotization and scapolitization, suggesting that H 2 O- and chlorine were essential in the formation. Furthermore, the pyroxene of the pegmatite shows a more distinct negative Eu anomaly than the pyroxene of the host gabbronorite. Amphibole dyklets associated with the unzoned pegmatite pockets consist of Cl-rich pargasite and Cl-scapolite, indicating that the fluid pressure during crystallization of the unzoned pegmatites exceeded the confining pressure and resulted in the fracturing of host rocks and expulsion of a Cl- and H 2 O-rich fluid and residual melt that subsequently formed amphibole veins. The zoned pegmatites of the mafic-ultramafic Hamn intrusion are internally differentiated in terms of grain size, texture, and mineral composition. A characteristic comb-like diopside layering shows a change from numerous small to fewer larger grains from the rim towards the core of the pegmatite pocket. This textural change suggests that the pegmatite-forming melt experienced a transition from a high nucleation rate (N) vs. growth rate (G) ratio to a low N/G ratio. A decreasing degree of undercooling with continuous crystallization from the rim to the core of the pockets can explain this transition. Apart from the comb-like layering, the bifurcating texture of some of the diopside grains is another evidence that the pegmatite-forming melt experienced undercooling. The pegmatites could have formed from a remobilized intercumulus or fractionated melt that was emplaced as residual melt into the colder host rock. Fast heat diffusion towards the host rock could have caused undercooling of the pegmatite-forming melt, which led to the formation of the comb-like and partly extremely coarse-grained texture. In contrast to the unzoned pegmatites, the zoned pegmatites lack evidence of significant involvement of chlorine- and H 2 O. Instead, the fluid inclusion study revealed that the pegmatite-forming melt was enriched in CO 2 . Microthermometry of the CO 2 -bearing inclusions indicates a minimum formation pressure of 647 to 734 MPa, and the titanium-in-quartz geothermometer yields a minimum formation temperature of 753 ± 34 °C for the quartz segregation. • The mafic-ultramafic Hamn intrusion hosts unzoned and zoned gabbroic pegmatites. • The unzoned pegmatites crystallized from a H 2 O and Cl rich mafic melt. • Change in degree of undercooling led to a comb-like texture of the zoned pegmatites. • The zoned pegmatites crystallized from a dry but CO2-enriched mafic melt.

Zircon Dating, Geochemistry, and Metallogenic Significance of Early Paleozoic Mafic Rocks in Northern Guangxi Province, China

Magmatic rocks, deformed structures, and tin–polymetallic deposits are widely distributed in the western Jiangnan Orogenic Belt (JOB) of northern Guangxi Province, China. Magmatism and mineralization are believed to have occurred in the Neoproterozoic stage. Herein, we report the zircon U–Pb dating results of investigations on the Ping’an Pb–Zn–Cu polymetallic veins hosted in gabbro near Ping’an Village. Zircon U–Pb dating of the host gabbro yielded ages of 853.0 ± 7.8, 837.7 ± 7.2, and 450.4 ± 6.7 Ma. The younger age represents the emplacement of gabbros, whereas the older Neoproterozoic age reflects inherited zircons from the wall rocks or source regions. The formation of gabbros (Caledonian) is related to the subduction and collisions of microplates in the western JOB, which are controlled by movements of the Yangtze and Cathaysian plates. We consider that the late Caledonian regional shearing in the western JOB resulted in the fracturing and faulting of rocks (Neoproterozoic and early Caledonian), which provided conduits for the flow of hydrothermal fluids and accommodation for the associated mineralization. Geochemistry investigations show that the Caledonian basic magmatic activity provided a certain material source for the final mineralization. We propose that the tin–polymetallic deposits in the northern Guangx Province, and Neoproterozoic cassiterite crystallization, were subjected to Caledonian shear crushing and hydrothermal transformation with copper, lead, zinc, and other metal elements based on our comprehensive analysis, providing a new ideology for understanding the geology and mineralization in this area.

Potassium-rich granite melt inclusions in zircon from gabbro-hosted felsic stringers, Mid-Atlantic Ridge at 13°34′N: E-MORB connection

We present new data on crystallized micro-inclusions in zircon from clinopyroxene-bearing felsic stringers within gabbronorite of gabbro-peridotite oceanic core complex at Mid-Atlantic Ridge, 13°34′ N. A relatively large (about 49 × 22 μm) crystallized inclusion exposed in situ in thin section consists of the muscovite + biotite + plagioclase + K-feldspar + quartz assemblage. Smaller inclusions (≤20 μm) found in zircon separates consist of silicate minerals, interstitial water solution, and gas bubbles. At temperature close to 800 °C and estimated pressure about 1.5 kbar the inclusions completely homogenize into a K2O-rich (3.9 ± 0.1 wt%), H2O-bearing (about 4 wt%) granite (SiO2 70 ± 2 wt%) melt. Calculations employed DOMINO software show that complete closed-system crystallization of the analyzed melt exactly reproduces mineral assemblage of the large crystallized inclusion (+H2O) at about 1.5 kbar below 600 °C. Granitoid mineralogy of felsic stringers hosting zircon does not correspond to that of the crystallized melt inclusions, with a significantly lower K2O in the former. The K2O loss might result from interaction of the melt with pyroxenes of the host gabbro. This implies that considering felsic granitoid veins, often present in the oceanic lower crustal sections, as simply “frozen melt” is not always justified. Compositional variations of basalt lava glasses from the 13°25′–14° portion of the Mid-Atlantic Ridge show continuous geochemical trends from D-MORB to E-MORB with K2O enrichment up to 0.9 wt%, suggesting mixing from two sources. The most K2O-enriched E-MORB composition (K2O = 0.9 wt%, H2O = 0.7 wt%, K2O/TiO2 = 0.42, (La/Sm)N = 2.45) was assumed as a source of the granitoid inclusions. Computer simulation of equilibrium crystallization of this E-MORB composition returned a granite residual melt closely matching the average composition of the zircon-hosted melt inclusions at 820 °C, 1.5 kbar and melt fraction 5.2 vol%. The result suggests a direct link between the E-MORB magma occurring in the studied area, and the origin of the K-rich granite melt in zircon-hosted inclusions.

MORB Melt Transport through Atlantis Bank Oceanic Batholith (SW Indian Ridge)

AbstractThe Atlantis Bank Oceanic Batholith is a 660 km2 gabbro massif representing the plutonic foundation of a major ridge magmatic center. It was continuously accreted, emplaced, and exposed in the rift mountains of the paleo-SW Indian Ridge from 13 to 10·3 Ma. Ocean Drilling Program (ODP) Hole 735B, drilled to 1508 m at Atlantis Bank, recovered evolved intercalated olivine and oxide gabbros representing the upper levels of the lower ocean crust. Within this section, ∼5·6 m of primitive chromian-spinel-bearing troctolites (0·1–2 m thick), with sharp modal contacts with the host gabbros, were cored between 410 and 500 m depth. Here we present new mineral (chromian spinel, clinopyroxene, olivine, and plagioclase) major and trace element and petrographic data for the troctolite suites (i.e. troctolites, clinopyroxene-rich troctolites, troctolitic gabbros) from 410 to 528 mbsf (meters below seafloor) and olivine gabbros in the 0–274 mbsf and 410–500 mbsf intervals, and examine the origin of these troctolite layers. Equilibrium melts for the troctolites are primitive to moderately evolved, with Mg# [= 100Mg/(Mg + Fe), 48–68 mol%] comparable with those of Atlantis Bank basalts. By contrast, equilibrium melts for both the 0–274 mbsf and 410–500 mbsf host olivine gabbros are highly to moderately evolved (Mg# 27–63 mol%). The 410–500 mbsf troctolite suites maintain high clinopyroxene Mg# (81–89 mol%) and mineral concentrations of compatible elements (i.e. clinopyroxene Cr2O3 0·1–1·2 wt%, Ni 151–330 μg g–1 and olivine Ni 1055–1559 μg g–1) with the increase in incompatible trace element abundances (i.e. clinopyroxene TiO2 0·3–2·1 wt%, Zr 5·8–112 μg g–1 and olivine Ti up to 293 μg g–1). Combined with abundant dissolution–reprecipitation textures, our results indicate that the troctolites were formed by the reaction between a spinel-bearing, troctolitic mush, from which they inherited the high Mg#, Ni and Cr, and percolating melts adding incompatible trace elements. Moreover, the spinel (NiO versus TiO2) and olivine (Ti versus Y) trace element compositions indicate that the troctolites were affected by low-degree Fe–Ti-rich melt metasomatism. In contrast, both the 0–274 mbsf and 410–500 mbsf olivine gabbros display a prominent decrease in clinopyroxene Mg# (from 88 to 66) and mineral compatible element concentrations (i.e. clinopyroxene Cr2O3 from 1·1 to ∼0 wt%, Ni from 208 to 34 μg g–1 and olivine Ni from 1200 to 136 μg g–1) with increasing incompatible trace element abundances (e.g. clinopyroxene Zr from 4·8 to 157 μg g–1). These features are compatible with the reactive porous flow of slightly to highly evolved melts through the cooling crystal mush zone. Our results, combined with the literature data, indicate that most olivine gabbros between 410 and 500 mbsf were formed prior to the troctolite layers. We document that the troctolites represent conduits for mid-ocean ridge basalt melt transport through the lower oceanic crust, whereas the olivine gabbros represent crystallization of a large crystal mush, recording initial gabbro emplacement, hyper- and sub-solidus deformation, and melt–rock reaction owing to upward penetrative flow of intercumulus melt.

Fossil oceanic core complexes in the Alps. New field, geochemical and isotopic constraints from the Tethyan Aiguilles Rouges Ophiolite (Val d\u2019H\xe9rens, Western Alps, Switzerland)

Exhumation of basement rocks on the seafloor is a worldwide feature along passive continental margins and (ultra-) slow-spreading environments, documented by dredging, drilling or direct observations by diving expeditions. Complementary observations from exhumed ophiolites in the Alps allow for a better understanding of the underlying processes. The Aiguilles Rouges ophiolitic units (Val d’Hérens, Switzerland) are composed of kilometre-scale remnants of laterally segmented oceanic lithosphere only weakly affected by Alpine metamorphism (greenschist facies, Raman thermometry on graphite: 370–380 °C) and deformation. Geometries and basement-cover sequences comparable to the ones recognized in actual (ultra-) slow-spreading environments were observed, involving exhumed serpentinized and carbonatized peridotites, gabbros, pillow basalts and tectono-sedimentary cover rocks. One remarkable feature is the presence of a kilometric gabbroic complex displaying preserved magmatic minerals, textures and crosscutting relationships between the host gabbro and intruding diabase, hornblende-bearing dikelets or plagiogranite. The bulk major and trace element chemistry of mafic rocks is typical of N-MORB magmatism (CeN/YbN: 0.42–1.15). This is supported by in-situ isotopic signatures of magmatic zircons (εHf = + 13 ± 0.6) and apatites (εNd = + 8.5 ± 0.8), determined for gabbros and plagiogranites. In-situ U–Pb dating was performed on zircons by laser ablation-ICP-MS, providing ages of 154.9 ± 2.6 Ma and 155.5 ± 2.8 Ma, which are among the youngest for oceanic gabbros in the Alps. Our study suggests that the former Aiguilles Rouges domain was characterized by tectonism and magmatism resembling present-day (ultra-) slow-spreading seafloor. It also suggests that the Tethyan lithosphere is laterally segmented, with punctuated magmatism such as the Aiguilles Rouges gabbros and carbonated ultramafic seafloor covered by basalts and Jurassic tectono-sedimentary deposits.

Sr-Nd-Hf-O isotope constraints on crustal contamination and mantle source variation of three Fe-Ti-V oxide ore deposits in the Emeishan large igneous province

Magma-carbonate interaction and recycled oceanic crust in the mantle source are thought to have played a critical role in the genesis of magmatic Fe-Ti-V oxide ore deposits in the Emeishan mantle plume-related large igneous province in southwestern China. To test these hypotheses, we have carried out a combined study of zircon Hf-O isotopes and whole-rock Sr-Nd isotopes on three representative magmatic Fe-Ti-V oxide ore deposits associated with different types of wall-rock in the province: the Hongge and Taihe deposits with granitoid wall-rocks in most places, and the Panzhihua deposit with marble wall-rocks. Diopside-garnet marble xenoliths are common in the host gabbros of the Taihe deposit. The zircon separates used in this study are all from the mineralized units of the host layered mafic–ultramafic intrusions. The selected zircon grains yield U-Pb isotopic ages of 258.2 ± 2.8 Ma for Hongge intrusion, 263.3 ± 2.2 Ma for Taihe intrusion, and 257.6 ± 2.1 Ma for Panzhihua intrusion. The Hf-O isotopes of the zircon separates are εHf(t) from 3.1 to 4.7 and δ18O from 5.77 to 6.45‰ for Hongge, εHf(t) from 8.3 to 9.8 and δ18O from 4.12 to 4.56‰ for Taihe, and εHf(t) from 4.6 to 6.3 and δ18O from 5.49 to 6.10‰ for Panzhihua. The zircon δ18O values of the Taihe deposit are significantly lower than the mantle zircon values (5.3 ± 0.3‰), whereas those of the Hongge and Panzhihua deposits are higher than the mantle values. The initial 87Sr/86Sr and εNd(t) of the whole rock samples from the deposits are similar, from 0.7047 to 0.7052 and from 2.3 to 3.1, respectively. The observed decoupling of zircon Hf-O isotopes (low δ18O and high εHf(t) values) in the Taihe deposit is most consistent with a high-temperature altered, low δ18O oceanic gabbroic component in the plume source for this deposit. All of the isotope data together show that the parental magmas for these deposits experienced different types of crustal contamination. Contamination with the Precambrian metamorphic country rocks is much more significant in the parental magma for the Hongge deposit than those for the Panzhihua and Taihe deposits. On the other hand, contamination with marble country rocks is more pronounced in the parental magmas for the Panzhihua and Taihe deposits than in the parental magma for the Hongge deposit. An important new finding from this study is that these deposits are not linked to the same component of the mantle plume nor a single type of crustal contamination.

The Ediacaran post-collisional Khamal gabbro-anorthosite complex from the Arabian Shield and its Fe-Ti-P ore: An analogy to Proterozoic massif-type anorthosites

The Khamal complex is an Ediacaran post-collisional gabbro-anorthosite igneous complex that intruded Tonian and Cryogenian subduction-related island arc and ophiolitic rocks and syn-tectonic granitic rocks. The complex is reminiscent of massif-type anorthosites. It has an elliptical shape and is unmetamorphosed and undeformed. The Khamal complex is differentiated into four lithological units, marginal gabbro, anorthosite, central gabbro and northern gabbro. Anorthosite is the most dominant unit and is surrounded by marginal high-Al (Al2O3 >16 wt%) gabbro and gabbronorite showing a positive Eu anomaly. Iron-rich gabbroic rocks associated with nelsonite (Fe-Ti-P ore) and magnetite-ilmenite (FeTi ore) are exposed in the central portion of the anorthosite. Towards the north, less differentiated massive gabbronorite is exposed, which is characterized by its high Al2O3 contents in addition to the MgO contents. The association between anorthosite and AlFe mafic melts is recorded in several Proterozoic massif-type anorthosite complexes. The formation of the Khamal gabbro-anorthosite complex was most likely related to the waning stage of the East African orogeny. After the assembly of the Tonian-Cryogenian island arc terranes, slab break-off and lithospheric delamination induced asthenospheric upwelling and ponding of the mafic melt at the crust-mantle boundary. The resulting mantle-derived or lower crustal melt evolved, which led to the formation of floated plagioclase. Then, the plagioclase-rich diapir ascended to form the anorthosite and high-Al marginal gabbroic rocks. The less differentiated high-Al gabbronorite of the northern unit intruding the anorthosite mimics its parental magma but does not directly represent the magma from which anorthosite crystallized. The central gabbroic unit is characterized by an enrichment in iron. This unit might represent the residual melt that was extracted and segregated into a small chamber from the anorthosite, which was emplaced in a dynamic environment. In this chamber the melt underwent further crystal fractionation and accumulation processes. The compositional gap between the nelsonite and magnetite-ilmenite ores and the enclosing gabbroic rocks as well as the similarity between these ores and the host gabbro and the discordant nature of the FeTi ore favor the immiscibility origin of the ore.

Emplacement and High‐Temperature Evolution of Gabbros of the 16.5°N Oceanic Core Complexes (Mid‐Atlantic Ridge): Insights Into the Compositional Variability of the Lower Oceanic Crust

AbstractThis study reports the composition of the oceanic crust from the 16.5°N region of the Mid‐Atlantic Ridge, a spreading ridge segment characterized by a complex detachment fault system and three main oceanic core complexes (southern, central, and northern OCCs). Lithologies recovered from the core complexes include both greenschist facies and weathered pillow basalt, diabase, peridotite, and gabbro, while only weathered and fresh pillow basalt was dredged from the rift valley floor. The gabbros are compositionally bimodal, with the magmatic crust in the region formed by scattered intrusions of chemically primitive plutonic rocks (i.e., dunites and troctolites), associated with evolved oxide‐bearing gabbros. We use thermodynamic models to infer that this distribution is expected in regions where small gabbroic bodies are intruded into mantle peridotites. The occurrence of ephemeral magma chambers located in the lithospheric mantle enables large proportions of the melt to be erupted after relatively low degrees of fractionation. A large proportion of the dredged gabbros reveal evidence for deformation at high‐temperature conditions. In particular, chemical changes in response to deformation and the occurrence of very high‐temperature ultramylonites (&gt;1000 °C) suggest that the deformation related to the oceanic detachment commenced at near‐solidus conditions. This event was likely associated with the expulsion of interstitial, evolved magmas from the crystal mush, a mechanism that enhanced the formation of disconnected oxide‐gabbro seams or layers often associated with crystal‐plastic fabrics in the host gabbros. This granulite‐grade event was soon followed by hydrothermal alteration revealed by the formation of amphibole‐rich veins at high‐temperature conditions (~900 °C).

Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.

The Eocene corundum-bearing rocks in the Gangdese arc, south Tibet: Implications for tectonic evolution of the Himalayan orogen

The genesis of Liangguo corundum deposit in the southern Gangdese magmatic arc, east-central Himalaya, remains unknown. The present study shows that the corundum-bearing rocks occur as lenses with variable sizes in the Eocene gabbro that intruded into marble. These corundum-bearing rocks have highly variable mineral assemblage and mode. The corundum-rich rocks are characterized by containing abundant corundum, and minor spinel, ilmenite and magnetite, whereas the corundum-poor and corundum-free rocks have variable contents of spinel, plagioclase, sillimanite, cordierite, ilmenite and magnetite. The host gabbro shows variable degrees of hydration and carbonization. The corundum grains are mostly black, and rarely blue, and have minor FeO and TiO2. The spinel is hercynite, with high FeO and low MgO contents. The corundum-bearing rocks have variable but high Al2O3, FeO and TiO2, and low SiO2 contents. Inherited magmatic and altered zircons of the corundum-bearing rocks have similar U–Pb ages (∼47 Ma) to the magmatic zircons of the host gabbro, indicating corundum-bearing rock formation immediately after the gabbro intrusion. We considered that emplacement of gabbro induced the contact metamorphism of the country-rock marble and the formation of silica-poor fluid. The channeled infiltration of generated fluid in turn resulted in the hydrothermal metasomatism of the gabbro, which characterized by considerable loss of Si from the gabbro and strong residual enrichment of Al. The metasomatic alteration probably formed under P–T conditions of ∼2.2–2.8 kbar and ∼650–700 °C. We speculate that SiO2, CaO and Na2O were mobile, and Al2O3, FeO, TiO2 and high field strength elements remained immobile during the metasomatic process of the gabbro. The Liangguo corundum deposit, together with metamorphic corundum deposits in Central and Southeast Asia, were related to the Cenozoic Himalayan orogeny, and therefore are plate tectonic indicators.

Important role of hornblende fractionation in generating the adakitic magmas in Tongling, Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

ABSTRACTTongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of −3.00 to −4.52 and initial 87Sr/86Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.

Geology, U–Pb zircon geochronology, and geochemistry of PGE-bearing Neoarchean and Paleoproterozoic gabbroic rocks of the Peter Lake domain, southern Hearne craton, Canada

The Peter Lake domain, a component of the Hearne craton in Saskatchewan, includes several intrusive complexes ranging in age from 2.70 to 1.83 Ga. Two gabbroic complexes (Swan River and Porter Bay) together represent one of the largest accumulations of Precambrian mafic intrusion, both metamorphosed at amphibolite facies. Sulfide-poor PGE occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact-type PGE–(Ni–Cu) deposits such as the chaotic Lac des Iles pluton and with layered stratiform deposits of large layered intrusions such as Bushveld or Stillwater. Lithologically, both complexes are dominated by leucocratic gabbronorite and gabbro, locally characterized by cumulate layering, cross-bedding, brecciated, and pegmatitic textures. U–Pb zircon crystallization ages between 2562 and 2560 Ma were obtained for the Swan River complex. sulfide formation is interpreted to be of magmatic origin rather than remobilized, as had been speculated by exploration geologists, and is therefore of the same age as the host gabbro. Geochemical data support the interpretation of a mantle plume origin in a subduction environment for the Swan River complex. The Porter Bay complex is much more restricted in areal extent and a leucocratic gabbronorite yielded a U–Pb zircon age of 1913 ± 1 Ma, which is interpreted as the crystallization age and the time of sulfide formation. Major and trace element geochemical data of Porter Bay complex rocks show a considerably more evolved character than the Swan River complex, and is interpreted as indicating emplacement in a continental arc environment.

New Mineral NamesAntipiniteAnzaite-(Ce)BettertoniteBobshannoniteCalcinaksiteKhvoroviteLeguerniteLukkulaisvaaraiteMinjiangiteMöhniteMoraskoiteNickelpicromeritePilawite-(Ce)RalphcannoniteYeomanite

This New Mineral Names has entries for 15 new minerals, including antipinite, anzaite-(Ce), bettertonite, bobshannonite, calcinaksite, khvorovite, leguernite, lukkulaisvaaraite, minjiangite, mohnite, moraskoite, nickelpicromerite, pilawite-(Ce), ralphcannonite, and yeomanite. # Antipinite* {#article-title-2} N.V. Chukanov, S.M. Aksenov, R.K. Rastsvetaeva, K.A. Lysenko, D.I. Belakovskiy, G. Farber, G. Mohn and K.V. Van (2015) Antipinite, KNa3Cu2(C2O4)4, a new mineral species from a guano deposit at Pabellon de Pica, Chile. Mineralogical Magazine, 79(5), 1111–1121. Antipinite (IMA 2014-027), ideally KNa3Cu2(C2O4)4, was discovered in the guano deposit at the lower part of the northern slope of Pabellon de Pica Mountain, Tarapaca Region, Chile. The new mineral honors Mikhail Yuvenal’evich Antipin (1951–2013), a Russian specialist in the crystallography and crystal chemistry of organometallic and coordination compounds. Antipinite occurs in association with halite, salammoniac, chanabayaite, and joanneumite. The host gabbro consists of amphibole, plagioclase, minor clinochlore, and accessory chalcopyrite. Antipinite forms isometric and short prismatic crystals up to 0.1×0.1×0.15 mm, and random aggregates up to 0.6 mm across. The new mineral is blue, translucent, with pale blue streak and a vitreous luster. It is brittle, with a Mohs hardness ~2. The cleavage is medium in three directions, which were not determined. D meas = 2.53(3) and D calc = 2.549 g/cm3. Antipinite does not fluoresce under UV radiation. It is optically biaxial (+), α = 1.432(2), β = 1.530(2), and γ = 1.698(5) (λ = 589 nm); 2 V meas …

New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy)

The mid-ocean ridge (MOR)-type gabbroic sequences from the Internal Ligurian ophiolites include olivine-rich troctolite lenses up to tens of metres thick. For one of these lenses, a portion of metre-scale thickness characterized by skeletal to dendritic olivines (harrisites) was observed near the contact with host gabbros. Spinels from the olivine-rich troctolites locally include Ti-pargasite to kaersutite frequently associated with phlogopite to Na-phlogopite. Spinel-hosted amphibole displays rare earth element (REE) patterns characterized by a negative Eu anomaly, thereby recording a magmatic process associated with plagioclase segregation. The amphiboles show variable depletion of light REE (LREE) relative to middle REE (MREE), and heavy REE (HREE) that are weakly enriched to depleted with respect to MREE. Crystallization of the inclusion-bearing spinels is attributed to cooling of hybrid melts that originated by interaction between primitive melts and gabbro-related melts relatively rich in SiO 2 and incompatible elements. Clinopyroxene and amphibole from the harrisites show extensive variability for trace element compositions, albeit characterized by subparallel incompatible element patterns. This chemical variability was most probably acquired in response to rapid crystal growth related to undercooling of the harrisite parental melt. We propose that the melt undercooling was related to interaction of a primitive melt batch with a gabbroic crystal mush. Supplementary material : Details of the analytical technique are available at http://doi.org/10.6084/m9.figshare.c.3284312

Intermetallic compounds, copper and palladium alloys in Au–Pd ore of the Skaergaard pluton, Greenland

Copper–palladium intermetallic compounds and alloys (2314 grains) from the Au–Pd ore of the Skaergaard layered gabbroic pluton have been studied. Skaergaardite PdCu, nielsenite PdCu3, (Cu,Pd)β, (Cu,Pd)α, (Pd,Cu,Au,Pt) alloys, and native palladium have been identified as a result of 1680 microprobe analyses. The average compositions and various chemical varieties of these minerals are characterized, as well as vertical and lateral zoning in distribution of noble metals. The primary Pd–Cu alloys were formed within a wide temperature interval broadly synchronously with cooling and crystallization of host gabbro and in close association with separation of Fe–Cu sulfide liquid. In the course of crystallization of residual gabbroic melt enriched in iron, noble and heavy metals and saturated with the supercritical aqueous fluid, PGE and Au are selectively concentrated in the Fe–Cu sulfide phase as Pd–Cu and Cu–Au alloys.

Petrogenesis of Early Cretaceous alkaline dolerite dykes in the Dabie Orogen, China: Constraints on the timing of lithospheric thinning

The Dabie Orogen in central China has long been proposed to transform from Triassic compression related to northward deep subduction to Late Jurassic to Early Cretaceous extension involving marked crustal thinning and doming processes. Zircon U–Pb ages, mineral chemistry, and whole-rock geochemistry of alkaline dykes intruding Early Cretaceous intermediate–mafic–ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension. These dykes of dominantly dolerites with minor monzodiorites have zircon U–Pb ages of 128.3–129.7 Ma and are characterized by Sr–Nd isotopic ratios around the enriched mantle I. The dolerites can be divided into two groups, with the first group having lower incompatible elements but more enriched Sr–Nd isotopes than the second group. Equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation. Systematic normative minerals, trace elements and isotopic contrasts of Early Cretaceous gabbros (125–130 Ma), dolerite dykes of this study, and Early Cretaceous mafic dykes (111–115 Ma) indicate that the source regions for the dolerites are deeper than those for the host gabbros. Therefore, the lithospheric mantle thickening probably continued until the formation of the dolerites, and the lithospheric mantle thinning occurred significantly later than previously proposed. The thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125–130 Ma but changed to less than that for the source region of tholeiitic basalts before 115 Ma, suggesting a minimum decrease of ~ 20 km within ~ 10 m.y. This rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 Ma.

The magmatic–hydrothermal transition in the lower oceanic crust: Clues from the Ligurian ophiolites, Italy

The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm–Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm–Nd isochron corresponding to an age of 154±20Ma and an initial εNd of 9.2±0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02–0.17wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5ppm) and δ18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.