High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand

Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high‐temperature (HT) to ultrahigh‐temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO₂ = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. The cause of Late Cretaceous HT‐UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore‐running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT–UHT conditions in the lower crust with little or no sign of this event at the surface.     

Genetic and ore-forming ages of the Fe–P–(Ti) oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block,NW China

Abstract

During the past 50 years, many geological and ore-deposit investigations have led to the discovery of the Fe–P–(Ti)-oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block, northeastern Tarim Craton. In this paper, we discuss the genetic and ore-forming ages, tectonic setting, and the genesis of these deposits (Kawuliuke, Qieganbulake and Duosike). LA-ICP-MS zircon U–Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U ages of 811?±?5?Ma, 811?±?4?Ma, and 840?±?5?Ma for Kawuliuke ore-bearing pyroxenite, Qieganbulake gabbro and Duosike ore-bearing pyroxenite, respectively. The CL images of the Kawuliuke apatite grains show core–rim structure, suggesting multi-phase crystallisation, whereas the apatite grains from Qieganbulake and Dusike deposits do not show any core–rim texture, suggesting a single-stage crystallisation. LA-ICP-MS apatite ²⁰⁷Pb-corrected U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 814?±?21?Ma and 771?±?8?Ma for the Kawuliuke ores, and 810?±?7?Ma and 841?±?7?Ma for Qieganbulake and Duosike ores, respectively. The core–rim texture in apatite by CL imaging as well as two different ore-forming ages in the core and rim of the apatite indicate two metallogenic events for the Kawuliuke deposit. The first metallogenic period was magmatic in origin, and the second period was hydrothermal in origin. The initial ore-forming age of the Kawuliuke Fe–P–Ti mineralisation was ca 814?Ma and the second one was ca 771?Ma. On the other hand, the ore-forming ages of the Qieganbulake and Duosike deposits were ca 810?Ma and ca 841?Ma, respectively. Qieganbulake and Duosike deposits were of magmatic origin. Combined with previous geochronological data and the research on the tectonic background, we infer that the Kawuliuke, Qieganbulake and Duosike Fe–P–(Ti)-oxide deposits were formed in a subduction-related tectonic setting and were the product of subduction-related magmatism.     

Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°-79°E, 68°-70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south-west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south-west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth. This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre-metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion-driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.     

怀安蔓菁沟早前寒武纪高压麻粒岩混杂岩带地质特征、岩石学和同位素年代学

华北克拉通北缘中段怀安蔓菁沟高压麻粒岩混杂岩带产在太古宙怀安杂岩南缘与花岗岩带交界处,由高压基性麻粒岩、辉长质麻粒岩、英云闪长质麻粒岩和少量夕线石榴片麻岩相间排列的席状岩层构成,岩层间被高应变带或剪切带分隔。高压基性麻粒岩是石榴辉石麻粒岩。据石榴石斑晶内包裹的早期矿物(Cpx+Q)估算的早期高压变质作用条件:T=800℃,P>1.4GPa。环绕斑晶的后成合晶反应边矿物组合(P1+Opx+Hb+Cpx)的变质条件为:T=820℃,P为0.7～0.9GPa。全岩Sm-Nd等时线年龄2.65Ga,矿物Sm-Nd等时线年龄1.82Ga,锆石U-Pb一致线年龄1.83Ga。高压基性麻粒岩的原岩代表晚太古代陆壳的最下部,大约在2.7Ga从上地幔分异出来,可能经壳下垫托作用加在早期陆壳底部,随后经历高压变质作用。早元古代晚期,由于地壳规模的大型逆冲作用,使其上升,并经受褶皱形变、剪切推覆和退变质等作用的改造,形成高压麻粒岩混杂岩带。     

麻粒岩p—T—t轨迹的研究现状

综述了近年来麻粒岩中p-T-t轨迹的研究现状。内容涉及到麻粒岩相变质作用的温压条件及时间跨度,同时指出:结合岩石中的矿物组合、后成合晶及反应结构,利用矿物温压计是目前确定p-T-t轨迹的主要手段。     

变质流体研究某些重要进展

着重介绍了近年来国内外在变质流体与变质矿物共生组合，变质流体与变质反应温度，变质流体不混溶性以麻粒岩相变质流体特征研究方面所取得的一些重要进展。     

小莱河麻粒岩相铁建造的矿物组合—变质过程p、T变化的探讨

小莱河太古宙麻粒岩相铁建造的矿物组合有：①斜方辉石（Fs87）＋钙铁辉石＋石英＋磁铁矿±莱河矿；②铁闪石＋铁浅闪石质角闪石＋石英＋磁铁矿＋碳酸盐±Fe－镁川石（云辉闪石）。辉石的出溶显示了它形成的不同阶段：片晶发育的OpxⅡ－CpxⅡ是最后稳定产物，计算的平衡温度是742℃，按相图获得的压力是7×108～7．8×108Pa；钙铁辉石中“001”片晶是易变辉石片晶转变的，Opx－Cpx－Pig（片晶）阶段据相图推测可能形成于近820℃，8×108Pa的条件下。原始均一相的OpxⅠ－CpxⅠ阶段矿物成分按估算的片晶含量计算，它们的形成温度接近820℃，可能的压力范围是11×108～13×108Pa。由此得到ITD型麻粒岩相p－T－t轨迹。莱河矿无氧化成因的证据，认为它是在麻粒岩相条件下生成的。云辉闪石的Si—O链重复周期是3×，其Fe／（Fe＋Mg）＝0．85，是接近富铁端元的Fe－镁川石，它与闪石一起在640到近700℃条件下交代了辉石。     

Metamorphism and granite genesis in the Hidaka Metamorphic Belt, Hokkaido, Japan

The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island-arc type. The crust was formed during early Tertiary time, as a result of collision between two arc–trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I–IV), in which zone IV is in the granulite facies. A detailed study of the evolution of the Hidaka Belt, based on a revised P–T–t analysis of the metamorphic rocks, notably a newly found staurolite-bearing granulite, confirms a prograde isobaric heating path, after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33–40° C km^?1, and the highest P–T condition obtained from the lowest part of the granulite unit is 830° C, 7 kbar. In this part, X_H2O of Gt–Opx–Cd gneiss is about 0.15 and that of Gt–Cd–Bt gneiss is 0.4. The P–T–X_H2O condition of the granulite unit is well within a field where fluid-present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt–Opx–Cd gneiss in the lowest part of the granulite unit. The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S-type) granitic rocks within it. The S-type granitic rocks in this zone are Opx–Gt–Bt tonalite in the granulite zone, Gt–Cd–Bt tonalite in the amphibolite zone, and Cd–Bt–Mus tonalite in the Bt–Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S-type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust.     

Infiltration-driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

Dark hornblende + garnet-rich, quartz-absent metagabbro boudins from the Seguin subdomain, Ontario Grenville Province, are transected by anastomosing light-coloured veins rich in orthopyroxene, clinopyroxene, plagioclase and sometimes quartz. The veins vary in texture from fine-grained diffuse veins and patches that overprint the metagabbro, to coarse tonalitic leucosomes with sharp borders. The diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro, while the tonalitic leucosomes are suggestive of local internally-derived anatexis. All vein types grade smoothly into each other, with the tonalitic leucosomes being the latest.
Relative to the host metagabbro, the veins have higher Si, Na, Ba & Sr, lower Fe, Mg, Ca & Ti, and similar Al. The coarser veins are enriched in K. Plagioclase becomes steadily enriched in Na in the transition from host metagabbro (An₄₇) to the veins (An₃₅), and in the coarsest veins it is antiperthitic. Differences in composition of the other minerals between host metagabbro and vein are minor. Pressure–temperature estimates are scattered, but indicate a minimum temperature during vein formation of 700°C at about 8 kbar.
Mass balance constraints indicate that the veins formed from the metagabbro in an open system. The transecting veins are interpreted to represent pathways of Si + Na + Ba + Sr ± K ± Al-enriched, low a _H2O fluids that metasomatized the host metagabbro to form the anhydrous veins. An initial period of localized solid-state dehydration of the metagabbro, represented by the diffuse veins, was followed by a transition to localized anatexis, represented by the tonalitic leucosomes. The change to anatexis may have been due to the addition of K to the infiltrating fluid. The source and delivery mechanism of the fluids is unknown.     

Decompressional coronas and symplectites in granulites of the Musgrave Complex, central Australia

Abstract In granulite facies metapelitic rocks in the Musgrave Complex, central Australia, reaction between S1 garnet and sillimanite involves the development in S2 of both garnet + cordierite + hercynitic spinel + biotite and hercynitic spinel + cordierite + sillimanite + biotite. The S2 assemblages occur either in coronas and symplectites, mainly around garnet, or, in rocks in which S2 is more strongly developed, as recrystallized assemblages. Ignoring the presence of biotite and ilmenite, the mineral textures can be accounted for qualitatively by a consideration of the model system FeO-MgO-Al₂O₃-SiO₂ (FMAS); the textural relationships accord with decompression accompanying the change from S1 to S2. However, since biotite and ilmenite are involved in the assemblages, the parageneses are better accounted for in terms of equilibria in the expanded model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂-TiO₂-Fe₂O₃ (KFMASHTO), i.e. AFM + TiO₂+ Fe₂O₃. The coronas reflect the tectonic unroofing of at least part of the Musgrave Complex from peak S1 conditions of about 8 kbar to S2 conditions of about 4 kbar.