Response of detrital zircon and monazite,and their U–Pb isotopic systems,to regional metamorphism and host‐rock partial melting,Cooma Complex,southeastern Australia

Progressive Early Silurian low‐pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U–Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U–Pb ages survived through mid‐amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U–Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ～12°C/10⁶y from ～730 to ～170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600–500 Ma and 1.2–0.9 Ga old) indicate its derivation from a source remote from the Australian craton.     

Zircon Dating and Geological Implications of Granitic Gneiss in the Metamorphic Zone of Gaoligong Mountains in Western Yunnan, China

The general classification of intermediate-acid intrusive rocks in the metamorphic zone of Gaoligong Mountains as one of the metamorphic terranes of Proterozoic Gaoligong Mountains is problematic regarding the intrusion stage and age, as well as the subsequent metamorphism and deformation. In this study, we investigated granitic gneiss in the metamorphic zone of Gaoligong Mountains based on the 1:50,000 regional geological survey of Qushi Street (2011-2013) and SHRIMP U-Pb zircon geochronology. Results showed that the SHRIMP U-Pb zircon dating of granitic gneiss ranged from 163.5±5.7 Ma to 74.0±2.0 Ma. Thus, the granitic gneiss was grouped into orthometamorphic rocks (metamorphic intrusions). The dating data of granite rocks associated with intense metamorphism and deformation were divided into three groups, 163.5±5.7 to 162.3±3.1 Ma, 132.2-101.0 Ma and 99.4±3.5-74.0±2.0 Ma, which respectively represented three independent geologic events including an important magma intrusion with superimposed metamorphic effects in the late Middle Jurassic, regional dynamic metamorphism and superimposed reformation of fluid action in the early Cretaceous, and dynamic metamorphism dominated by ductile shear and metamorphism starting from the late Cretaceous.     

福建省泰宁县何宝山金矿床长兴岩体锆石LA-ICP-MS U-Pb年龄及其地质意义

长兴钾长混合花岗岩体与何宝山金矿床的成矿作用关系密切。何宝山金矿床的成矿作用表现出多期次、多阶段的特点,加里东晚期长兴岩体的侵入活动促进了金矿床成矿物质早期的迁移和富集,印支晚期—燕山早期的构造-岩浆活动叠加成矿。文中对区内加里东期主要侵入岩体进行了锆石LA-ICP-MS U-Pb同位素定年研究,得出长兴岩体的成岩年龄为(437.1±1.3)Ma,黑云母石英闪长岩成岩年龄为(436.6±1.1)Ma,黑云母花岗闪长岩脉成岩年龄为(427.1±1.4)Ma,从而确定金矿床成矿期上限,为进一步成矿作用研究提供科学依据。     

Dating mafic–ultramafic intrusions by ion-microprobing contact-melt zircon: examples from SW Sweden

Zircons from anatectic melts of the country rocks of three Proterozoic mafic–ultramafic intrusions from the Sveconorwegian Province in SW Sweden were microanalyzed for U–Th–Pb and rare earth elements. Melting and interaction of the wall rocks with the intrusions gave rise to new magmas that crystallized zircon as new grains and overgrowths on xenocrysts. The ages of the intrusions can be determined by dating this newly crystallized zircon. The method is applied to three intrusions that present different degrees of complexity, related to age differences between intrusion and country rocks, and the effects of post-intrusive metamorphism. By careful study of cathodoluminescent images and selection of ion probe spots in zircon grains, we show that this approach is a powerful tool for obtaining accurate and precise ages. In the contact melts around the 916?±?11?Ma Hakefjorden Complex, Pb-loss occurred in some U-rich parts of xenocrystic zircon due to the heat from the intrusion. In back-veins of the 1624?±?6?Ma Olstorp intrusion we succeeded in geochemically distinguishing new magmatic from xenocrystic zircon despite small age differences. At Borås the mafic intrusion mixed with country rock granite to form a tonalite in which new zircon grew at 1674?±?8?Ma. Reworking of zircon occurred during 930+33/–34?Ma upper amphibolite facies Sveconorwegian metamorphism. Pb-loss was the result of re-equilibration with metamorphic fluids. REE-profiles show consistent differences between xenocrystic, magmatic, and metamorphic zircon in all cases. They typically differ in Lu/La_N, Ce/Ce*, and Eu/Eu*, and igneous zircon with marked positive Ce/Ce* and negative Eu/Eu* lost its anomalies during metamorphism.     

Further evidence for an early Carboniferous (∼340 Ma) age of high-grade metamorphism in the Saxonian granulite complex

U–Pb and Pb–Pb zircon ages for metamorphic zircons from granulites in the Saxonian granulite complex are reported, using the SHRIMP ion microprobe, conventional multigrain and single-gain techniques and the evaporation method. This is complemented by a Pb–Pb evaporation age for a post-granulite granite emplaced into the schist mantle around the granulites during uplift of the complex. We also demonstrate that zircon ages are not reset during high-grade metamorphism, as commonly argued, but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation. Multifaceted zircons from four granulite samples that probably grew close to the peak of high-grade metamorphism yielded identical U–Pb and Pb–Pb ages of ～340?Ma which support previously published data and unambiguously show that the granulites formed during a lower Carboniferous event and not in the early Palaeozoic or Precambrian as previously suggested. Older cores in some of the metamorphic zircons reveal early Palaeozoic components at 470–485?Ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors. One sample suggests an inherited component as old as ～1700?Ma. The post-granulite granite has a Pb–Pb evaporation age of 333.1±1.0?Ma, and the short time interval between granulite metamorphism and granite intrusion implies that uplift, crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions.     

Precambrian to Paleozoic zircon record in the Siviez-Mischabel basement (western Swiss Alps)

U–Pb zircon analyses from three meta-igneous and two metasedimentary rocks from the Siviez-Mischabel nappe in the western Swiss Alps are presented, and are used to derive an evolutionary history spanning from Paleoarchean crustal growth to Permian magmatism. The oldest components are preserved in zircons from metasedimentary albitic schists. The oldest zircon core in these schists is 3.4 Ga old. Detrital zircons reveal episodes of crustal growth in the Neoarchean (2.7–2.5 Ga), Paleoproterozoic (2.2–1.9 Ma) and Neoproterozoic (800–550 Ma, Pan-African event). The maximum age of deposition for the metasedimentary rocks is given by the youngest detrital zircons within both metasedimentary samples dated at ~490 Ma (Cambrian-Ordovician boundary). This is in the age range of two granitoid samples dated at 505 ± 4 and 482 ± 7 Ma, and indicates sedimentation and magmatism in an extensional setting preceding an Ordovician orogeny. The third felsic meta-igneous rock gives a Permian age of intrusion, and is part of a long-lasting Variscan to post-Variscan magmatic activity. The zircons record only minor disturbance of the U–Pb system during the Alpine orogeny.     

The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)

This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U–Pb ages of zircon constrain the timing of regional amphibolite–granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U–Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900–930 °C and 1,000–1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700–800 °C and 810–870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750–800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust.     

柴北缘前寒武纪岩体（地层）变质年龄 及其对早古生代造山作用的指示

柴北缘前寒武纪岩体（地层）分布广泛。为确定柴北缘地区前寒武纪岩体（地层）受早古生代碰撞造山作用的影响,采用LA-ICP-MS技术,对大柴旦地区前寒武纪黑云斜长片麻岩、斜长角闪岩及石榴子石斜长角闪岩中的锆石进行了U-Pb同位素定年。黑云斜长片麻岩获得479~472Ma的变质年龄,斜长角闪岩获得440Ma和470Ma 2个变质年龄,石榴子石斜长角闪岩获得418.8Ma±3.0Ma的变质年龄。初步确定,柴北缘早古生代造山作用对前寒武纪岩体构成了3次强度不等的变质作用叠加,分别为大洋俯冲末期阶段(495~467Ma)岛弧花岗岩弱热烘烤变质作用、大陆碰撞造山阶段(467~423Ma)区域变质作用、S型花岗岩热动力变质作用和大陆后碰撞造山阶段(423~371Ma) I型花岗岩强烈接触热变质作用。     

Timing of the Yuchiling giant porphyry Mo system, and implications for ore genesis

The Yuchiling Mo deposit is a recently discovered giant porphyry system in the East Qinling Mo belt, China. Its apparent causative intrusion, i.e., the Yuchiling granite porphyry, is the youngest intrusion (phase 4) of the Heyu multiphase granite batholith, which was emplaced between 143 and 135 Ma. New robust constraints on the formation of the Yuchiling porphyry Mo system are provided by combined zircon U–Pb, biotite ⁴⁰Ar/³⁹Ar, and molybdenite Re–Os dating. Zircon grains from the Mo-mineralized granite porphyry yield weighted ²⁰⁶Pb/²³⁸U age of 134.0?±?1.4 Ma (n?=?19, 2σ error, MSWD?=?0.30). Magmatic biotite from the same sample yield a ⁴⁰Ar/³⁹Ar plateau age of 135.1?±?1.4 Ma (2σ error), and an inverse isochron age of 135.6?±?2.0 Ma (n?=?7, 2σ error, MSWD?=?10.8), which are effectively coincident with the zircon U–Pb age within analytical error. Three pulses of mineralization can be deduced from the molybdenite Re–Os ages, namely: ～141, ～137, and ～134 Ma, which agree well with the zircon U–Pb ages of granitic phases 1, 2, and the Yuchiling porphyry (phase 4), respectively. These well-constrained temporal correlations indicate that Mo mineralization was caused by pulses of granitic magmatism, and that the ore-forming magmatic-hydrothermal activity responsible for the Yuchiling porphyry Mo system lasted about 8 Ma. The Yuchiling Mo deposit represents a unique style of porphyry Mo system formed in a post-collision setting, and associated with F-rich, high-K calc-alkaline intrusions, which differ from convergent margin-associated porphyry Mo deposits.     

Two periods of mineralization in Xiaoxinancha Au–Cu deposit,NE China: evidences from the geology and geochronology

The Xiaoxinancha Au–Cu deposit is located at the eastern segment of the Tianshan–Xingmeng orogenic belt in northeast China. The deposit includes porphyry Au–Cu orebodies, veined Au–Cu orebodies and veined Mo mineralizations. All of them occur within the diorite intrusion. The Late Permian diorite, Late Triassic granodiorite, Early Cretaceous granite and granite porphyry are developed in the ore area. The studies on geological features show that the porphyry Au–Cu mineralization is related to the Late Permian diorite intrusion. New geochronologic data for the Xiaoxinancha porphyry Au–Cu deposit yield Permian crystallization zircon U–Pb age of 257 ± 3 Ma for the diorite that hosts the Au–Cu mineralization. Six molybdenite samples from quartz + molybdenite veins imposed on the porphyry Au–Cu orebodies yield an isochron age of 110.3 ± 1.5 Ma. The ages of the molybdenites coeval to zircon ages of the granite within the errors suggest that the Mo mineralization was genetically related to the Early Cretaceous granite intrusion. The formation of the diorite and the related Au–Cu mineralization were caused by the partial melting of the subduction slab during the Late Palaeozoic palaeo‐Asia Ocean tectonic stage. The Re contents and Re–Os isotopic data indicate that the crustal resource is dominated for the Mo mineralization during the Cretaceous extensional setting caused by the roll‐back of the palaeo‐Pacific plate. Copyright © 2014 John Wiley & Sons, Ltd.     

Evidence for a Caledonian amphibolite to eclogite facies pressure gradient in the Middle Allochthon Lindås Nappe,SW-Norway

The Proterozoic anorthosite–mangerite–charnockite complex dominating the Lindås Nappe in the Scandinavian Caledonides was locally eclogitized in the southwestern part of the nappe during the Caledonian orogeny, whereas only amphibolite facies assemblages are recorded in the rest of the nappe. Sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite-free exposures of the nappe exhibit large garnet phenoblasts (ca. 900°C) that are fractured and partly replaced by a Caledonian symplectitic amphibolite facies assemblage (ca. 515°C). Metamorphic zircon attributed to this garnet breakdown is dated by ID-TIMS U–Pb at 430 ± 3 Ma, suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south, probably implying that the section across the nappe represents a Caledonian pressure gradient. The rocks also preserve a complex Sveconorwegian history including an age of 969 ± 6 Ma, which we interpret as dating magmatic emplacement of jotunitic–anorthositic portions of the complex, 936 ± 12 Ma reflecting the granulite facies metamorphism, and 908 ± 16 Ma, representing a late generation of zircon best explained as having formed by metasomatic processes. Caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone, resetting their U–Pb systems, and forming new ones, hereby also demonstrating a case of resetting and recrystallization of low-U zircon. Our data, gained from diverse lithologies, illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the Lindås Nappe and the Jotun Nappe Complex.     

Permian age of the Wudaogou Group in eastern Yanbian: detrital zircon U–Pb constraints on the closure of the Palaeo-Asian Ocean in Northeast China

The Wudaogou Group in eastern Yanbian, Northeast China, plays a key role in constraining the timing and eastward termination of the Solonker–Xra Moron River–Changchun Suture, where the Palaeo-Asian Ocean closed. The Wudaogou Group consists of schist, gneiss, amphibolite, metasedimentary, and metavolcanic rocks, all of which underwent greenschist- to epidote–amphibolite-facies regional metamorphism, with some hornfels resulting from contact metamorphism. To determine the age of deposition, the timing and grade of metamorphism, and the tectonic setting of the Wudaogou Group, we investigated the petrography and geochronology of the metamorphic rocks in this group. Zircons from the metasedimentary rocks of this group can be divided into metamorphic zircons and detrital zircons of magmatic origin. U–Pb ages of metamorphic zircons dated by LA-ICP-MS vary from 249 ± 4 to 266 ± 4 Ma, approximating the age of regional metamorphism in the eastern Yanbian area. Detrital zircons yield U–Pb ages ranging from 253 ± 5 to 818 ± 5 Ma, and indicate that the provenance of the Wudaogou Group experienced four tectonic–thermal events between 818 and 253 Ma: Neoproterozoic (ca. 818–580 Ma), Cambro–Ordovician (ca. 500–489 Ma), Devonian–Carboniferous (ca. 422–300 Ma), and middle–late Permian (ca. 269–253 Ma). The youngest detrital zircon, with a U–Pb age of 253 ± 5 Ma, defines the maximum depositional age of the Wudaogou Group. The presence of the Cambro-Ordovician and Neoproterozoic detrital zircons implies that the source of the Wudaogou Group had an affinity with Northeast China, which leads us to conclude that the Solonker–Xra Moron River–Changchun Suture extends from Wangqing to Hunchun in eastern Yanbian, and that the Palaeo-Asian Ocean may have closed at the end of the Permian or Early Triassic period.     

Zircon U‐Pb/Lu‐Hf and monazite chemical dating of the Tirodi biotite gneiss: implication for latest Palaeoproterozoic to Early Mesoproterozoic orogenesis in the Central Indian Tectonic Zone

In this investigation, we reconstruct the latest Palaeoproterozoic to Early Mesoproterozoic orogenic events along the southern margin of the Central Indian Tectonic Zone (CITZ), using sensitive high resolution ion microprobe (SHRIMP) U‐Pb zircon dating and Lu‐Hf isotope analyses of zircon and Th‐U‐Pb chemical dating of monazite from samples of the Tirodi biotite gneiss (TBG) unit in the Sausar Mobile Belt (SMB), the latter constituting the southernmost litho‐tectonic component of the CITZ. U‐Pb zircon dating of one migmatitic gneiss sample from the type locality of the Tirodi biotite gneiss in the northern domain of the SMB has yielded an age of 1618 ± 8 Ma, which is considered to be the time of magmatic crystallization of its protolith. Combined U‐Pb zircon and monazite chemical dating of two granite gneiss samples from the southern domain of the SMB broadly constrain magmatic crystallization between 1603 ± 23 Ma and 1584 ± 17 Ma and an overprinting metamorphic recrystallization event at 1572 ± 7 Ma. Monazites from the granite gneiss samples also record a terminal metamorphic event at 1415 ± 23 Ma. Lu‐Hf isotopic analyses of zircons reveal fundamentally different source rock reservoirs for the protoliths of these magmatic rocks across the SMB. While the type TBG from the northern domain was derived from an Early Palaeoproterozoic source T(Hf) from 2093 to 2523 Ma, with a mean value at 2379 Ma) of essentially juvenile material with minor crustal components (εHf(t) from −3.3 to + 3.7), the granite from the southern domain had a mature crustal source (εHf(t) from −12.5 to −21.9) of Palaeoarchaean age T(Hf) from 3051 to 3630 Ma, with a mean value at 3218 Ma). When integrated with metamorphic information previously obtained from the 1.6 Ga ultra‐high temperature granulite facies metamorphic event in the SMB, the discrete magmatic and metamorphic events between 1.62/1.60 Ga and 1.42 Ga can be correlated with the formation of an Early Mesoproterozoic accretionary orogen in the CITZ. Copyright © 2011 John Wiley & Sons, Ltd.     

Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction–collision system,Bergen Arcs,Norway

Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.     

辽东明安硼镁矿床混合花岗岩的锆石U-Pb年龄及对成矿时代的制约

辽东硼矿赋存于辽吉裂谷中央凹陷带内的里尔峪组地层中,形成于火山活动与蒸发沉积交替旋回的环境,后期经历了低压角闪岩相至混合岩化变质作用,属沉积-变质型硼矿床,主矿区包括有后仙峪、翁泉沟和砖庙-杨木杆三处。混合岩化作用使裂谷早期喷发的酸性火山岩发生原地重熔,形成大量层状产出的混合花岗岩、变粒岩和浅粒岩。文章利用LA-MC-ICP-MS技术对砖庙-杨木杆矿区明安硼镁矿体下盘,与上部地层发生同构造变形的层状混合花岗岩进行了锆石U-Pb年龄的测定,尝试解决明安硼矿的成矿时代问题。混合花岗岩的锆石阴极发光图像显示,锆石具有核-边结构,核部的震荡环带为岩浆成因,边部为变质增生成因。锆石核部的U-Pb定年结果表明：²⁰⁷Pb/²⁰⁶Pb加权平均年龄为（2229±3）Ma（MSWD=0.8）,Th/U值为0.4479~1.1309。因此,笔者认为（2229±3）Ma年龄代表了层状混合花岗岩原岩——酸性火山岩的喷发时代,也代表了硼镁矿的初始含硼蒸发岩形成时代的下限。     

U–Pb zircon and monazite age constraints on granulite-facies metamorphism and deformation in the Strangways Metamorphic Complex (central Australia)

The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M ₁–M ₂ and the correlated periods of intense deformation (D ₁–D ₂) are part of a single tectonothermal event between 1,717±2 and 1,732±7 Ma. It is considered unlikely that the two metamorphic phases (M ₁, M ₂) suggested by earlier work represent separate events occurring within 10–25 Ma of each other. Previous higher estimates for the age of M ₁ granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U–Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U–Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield ²⁰⁷Pb/ ²³⁵U ages between 1,728±11 and 1,712±2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.     

The Plat Sjambok Anorthosite and its tonalitic country rocks: Mesoproterozoic pre-tectonic intrusions in the Kaaien Terrane,Namaqua–Natal Province,southern Africa

The Plat Sjambok Anorthosite crops out near Prieska Copper Mines in the Namaqua–Natal Province of southern Africa. It is a massif-type anorthosite, previously regarded as a late-tectonic intrusion and part of the ca. 1100 Ma bimodal Keimoes Suite. Our new ion probe U–Pb zircon data show that the Plat Sjambok massif intruded at 1259 ± 5 Ma, before the 1220 Ma Namaqua collision events and is thus approximately 150 million years older than the Keimoes Suite. Despite the proximity to Prieska Mines, the anorthosite is located in the Kaaien Terrane close to the Brakbos Fault, which is the boundary with the Areachap Terrane in which Prieska Mines is situated. We dated the Nelspoortjie Tonalite, the main country rock of the Plat Sjambok Anorthosite, by laser ablation ICPMS at 1273 ± 13 Ma. Both intrusions thus originated concurrently with the 1286–1241 Ma volcanic rocks of the Areachap Group, which developed in a subduction-related arc setting, prior to its collision with the Kaaien Terrane and Kaapvaal Craton. Metamorphic zircon rims in the Plat Sjambok Anorthosite give an age of 1122 ± 7 Ma, a time that corresponds to a quiet period in the Areachap Terrane. We propose a tectonic model in which formation of the Nelspoortjie Tonalite and Plat Sjambok Anorthosite was driven by intrusions from the mantle into a back-arc related tensional environment within the Kaaien Terrane, possibly situated above an Archaean crustal tongue. This led to heating in a thickened crustal setting in which the tonalite originated as a partial melt of amphibolite. The anorthosite then formed as a mixture of mantle-derived gabbro and Archaean crustal rocks, which explains the 2100–2600 Ma zircon–Hf crustal residence ages and the Sm–Nd trend towards an old crustal source. The anorthosite and its country rocks were only juxtaposed with the Prieska Copper Mining District by late-tectonic uplift and transpressional movements on the Brakbos Fault towards the end of the Namaqua tectogenesis.     

Geochronology of high-pressure mafic granulite dykes in SW Sweden: tracking the P–T–t path of metamorphism using Hf isotopes in zircon and baddeleyite

Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.     

Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that give 90.3 ± 2.2 Ma U–Pb concordia age. Between inherited core and the metamorphic rim, a thin, dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. Preservation of angular-shaped inherited core of 5–10 μm in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near-peak metamorphism. The Ostwald ripening process consuming zircon less than 5–10 μm is required to form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite sample may indicate the contribution of this process. Zircon less than 20 μm is confirmed to be an important sink of Zr in metatexites, and ca. 35-μm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr.     

Petrochronology of the migmatization event of the Xolapa Complex,Mexico, microchemistry and equilibrium growth of zircon and garnet

ABSTRACT

The number of migmatization events in the Xolapa Complex and their absolute age are controversial. U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry was performed on zircon grains from migmatites to investigate the age of different textural domains. Rare-earth element (REE) partition coefficients between zircon and garnet were compared with those established for different temperatures in order to test for equilibrium growth. Two age domains were identified. In one sample where zircon and garnet coexist, the outer zircon overgrowths yield a mean age of 54.16 ± 0.29 Ma (mean square weighted deviation (MSWD) = 3.5), whereas intermediate zones, between the core and outer overgrowths, yield an age of 122.7 ± 1.8 Ma (MSWD = 2.5). Partition coefficients were calculated for REEs between coexisting garnet (two different populations) and zircon using (1) the composition of ca. 54 Ma zircon overgrowths and garnet rims and (2) zircon intermediate zones together with garnet cores. The cores of small garnet grains (garnet A) may have grown in equilibrium with zircon domains of ca. 122 Ma. Both garnet cores and rims of the larger porphyroblasts (garnet B) seem to be in equilibrium with ca. 54 Ma zircon overgrowths. Petrographic observations suggest that crystallization of garnet A occurred during partial melting, placing equilibrium growth and therefore a first migmatitic event during the Early Cretaceous at ca. 122 Ma. This migmatitic event may be related to the collision of the Chortís Block with western Mexico. A second migmatitic event of ca. 54 Ma is suggested by equilibrium growth of large garnets (group B) and the outer zircon overgrowths. The high geothermal gradient necessary for this second migmatitic event might be related to the exhumation of the Xolapa Complex, as a result of the transpression and tectonic transport of the Chortís Block to the southeast from the end of the Mesozoic to most of the Cenozoic.