Lead incorporation during crystal growth and the misinterpretation of geochronological data from low-238U/204Pb metamorphic minerals

The isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous, depends on the reaction history of the metamorphic rock, and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases. The initial lead composition has a significant effect on the age of low-²³⁸U/²⁰⁴Pb metamorphic phases (e.g. garnet, rutile, titanite, staurolite, vesuvianite and ilmenite). Using a distinct value (e.g. leached K-feldspar Pb, model Pb) rather than a geologically reasonably constrained range may result in apparently precise, yet inaccurate ages. Since age data from metamorphic minerals are widely used to unravel the P–T–t–d evolution of orogens, inaccurate ages result in: (1) incorrect timing (duration) of P–T loops and associated with it the heat budget and mass transfer in orogens; (2) arbitrary rates (based on the age difference between core and rim) for mineral growth, P–T evolution and deformation; and (3) apparent sequences of isotopic closure for the U–Pb system of contrasting minerals.     

Early Ordovician terrane accretion along the Gondwanian margin of the East Antarctic Craton: new Pb/Pb titanite ages from the Tonalite Belt, North Victoria Land, Antarctica

Early Palaeozoic subduction of the palaeo-Pacific plate and terrane accretion along the palaeomargin of the East Antarctic Craton is well-documented in North Victoria Land, where the Tonalite Belt is a complex of synkinematic intrusions emplaced within the Lanterman–Murchison Shear Zone at the boundary between the Wilson Terrane and the allochthonous Bowers Terrane. Stepwise leaching Pb/Pb and U–Pb studies of titanite separates carried out on two well-foliated samples of tonalites yielded ages of deformation bracketed between 490 and 480 Ma with an isochron age of 480 ±13 Myr. Ar/Ar and K–Ar ages of 477 Myr in the metamorphic rocks of accreted terranes point to fast cooling and uplift after accretion. The new titanite ages, compared with a regional distribution of magmatic and metamorphic ages, indicate an early Ordovician age for terrane collision and amalgamation. As a consequence of collision, subduction shifted to an outward position along the palaeomargin of the East Antarctic Craton.     

Petrology of ultrahigh-pressure rocks from the southern Su-Lu region, eastern China

Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.     

Early Ordovician U–Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Eclogite-grade metamorphism of the Seve Nappe Complex (SNC) in Norrbotten, Sweden, records the attempted subduction of the Baltic continental margin during the early Palaeozoic evolution of the Iapetus Ocean. Metamorphic titanite sampled from several calcsilicate gneisses of the SNC in Norrbotten occurs as part of a prograde, eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant U/Pb ages of 500–475  Ma. Later structural disruption of these rocks occurred during the Siluro-Devonian Scandian phase of the Caledonide orogeny, but the U/Pb systematics show no evidence of a second generation (metamorphic or recrystallized) of titanite, or of post-Early Ordovician disturbance through Pb loss. Hence the U/Pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the SNC.
These results support earlier Sm/Nd and ⁴⁰Ar/³⁹Ar studies indicating an Early Ordovician metamorphic age for the eclogitic Norrbotten SNC, and confirm the Early Ordovician destruction of at least this segment of the Palaeozoic passive margin of Baltica. These results indicate that the SNC in the northern Scandinavian Caledonides was subducted and metamorphosed to high grade some 50–70  Myr prior to the high-grade metamorphism of the SNC in the central Scandinavian Caledonides. This result requires significantly different early Palaeozoic tectonic histories for rocks mapped as SNC in the northern Caledonides and those in the central Caledonides, despite a seemingly similar tectonostratigraphic position and broadly similar high-grade metamorphism.     

Time constraints on the tectonic evolution of the Eastern Sierras Pampeanas (Central Argentina)

The application of the SHRIMP U/Pb dating technique to zircon and monazite of different rock types of the Sierras de Córdoba provides an important insight into the metamorphic history of the basement domains. Additional constraints on the Pampean metamorphic episode were gained by Pb/Pb stepwise leaching (PbSL) experiments on two titanite and garnet separates. Results indicate that the metamorphic history recorded by Crd-free gneisses (M2) started in the latest Neoproterozoic/earliest Cambrian (553 and 543 Ma) followed by the M4 metamorphism at ~530 Ma that is documented in the diatexites. Zircon ages of 492 Ma in the San Carlos Massif correlate partly with rather low Th/U ratios (<0.1) suggesting their growth by metamorphic fluids. This age is even younger than the PbSL titanite ages of 506 Ma. It is suggested that the fluid alteration relates to the beginning of the Famatinien metamorphic cycle in the neighbouring Sierra de San Luis and has not affected the titanite ages. The PTt evolution can be correlated with the plate tectonic processes responsible for the formation of the Pampean orogene, i.e., the accretion of the Pampean basement to the Río de La Plata craton (M2) and the later collision of the Western Pampean basement with the Pampean basement.     

Evolution of feldspars at the amphibolite-granulite-facies transition in augen gneisses (SW Norway): geochemistry and Sr isotopes

The Sveconorwegian Augen Orthogneisses of Rogaland — Vest-Agder (SW Norway) were emplaced as amphibole- and biotite-bearing granodiorites at 1040 Ma (concordant Rb/Sr and zircon U/Pb ages). They underwent prograde metamorphism which increased from lower amphibolite-facies in the eastern zone to granulite-facies in the western zone, close to the Rogaland anorthosite complex. K-feldspar megacrysts initially crystallised as phenocrysts and were chemically equilibrated during metamorphism, as shown by the flat Ba concentration profiles and the increase of the anorthite content from An_1.1 in the amphibolitefacies to An_2.6 in the granulite-facies. This increase of the An content suggests an increase in metamorphic temperature. The REE content of the megacrysts is related to the associated accessory minerals which depend upon the metamorphic grade: sphene + allanite + apatite + zircon and rarely thorite in amphibolite-facies, and apatite + zircon + monazite ± thorite in lower amphibolite-and granulite-facies. Amphibole and biotite inclusions in megacrysts were also equilibrated during metamorphism. Groundmass K-feldspar and plagioclase experienced late-metamorphic changes during uplift. An internal Rb/Sr mineral isochron (plagioclase, apatite, K-feldspar) defines an age of 870 Ma, which represents the closure of the Rb/Sr isotopic system in minerals of the augen gneisses. This age also represents a K-feldspar cooling age in regionally distributed augen gneiss samples. The K-feldspar cooling age appears to be similar to or slightly older than the biotite cooling age.     

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero:Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks.The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades.The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny,making it unique among dome-and-keel provinces.However,a lack of appropriate lithologies,datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province.A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak P-T conditions of 5-7 kbar and 700-750 ℃ and a published age of ca.2730 Ma based on U-Pb ages of zircon from leucosomes,suggesting that this age represents the migmatisation event.A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole,giving P-7 conditions of 8-9 kbar and 550 ℃ with an associated titanite age of ca.2050 Ma.A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700 ℃,and texturally late titanite from this sample produces an age of ca.2060 Ma.Three additional samples were collected from the edges of the dome.A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution.This sample has a U-Pb zircon age of ca.2775 Ma,which could date metamorphism or be the age of its volcaniclastic protolith.Texturally unconstrained titanite from the sample gives an age of ca.2040 Ma.A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca.2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700 ℃ on a near isobaric P-T path.An amphibolite dike,observed to cross-cut the felsic schist produces a zircon U-Pb age of ca.2760 Ma.Altogether this data suggests that the samples were metamorphosed in the Archean(ca.2775-2730 Ma)and again during the Transamazonian event.The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time.The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures,with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles.The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature,but they are susceptible to reworking,resulting in an enigmatic record of formation.     

西昆仑布伦阔勒岩群中泥质高压麻粒岩的岩石学、年代学及其地质意义

西昆仑的深变质岩类主要发育于布伦阔勒岩群之中,其中的高压麻粒岩是西昆仑造山带中目前已知的变质程度最高的岩石。本文以其中的泥质高压麻粒岩为研究对象,结合岩相学、相平衡模拟以及锆石年代学分析等方法进行研究。结果显示其峰期变质矿物组合蓝晶石+石榴石+钾长石,是典型的泥质高压麻粒岩岩石组合。根据相平衡模拟估算,高压麻粒岩相峰期变质的温压条件高于850℃及1.4GPa,退变质的温压条件约为650℃和0.6GPa。SHRIMP U-Pb锆石定年结果显示泥质高压麻粒岩记录了两期变质,第一期暗色变质锆石年龄为ca.185Ma,代表岩石从高压麻粒岩相峰期变质退变至近固相线阶段的年龄;第二期亮色变质增生边年龄为ca.166Ma,代表后期退变质年龄;而高压麻粒岩相峰期变质时代应在200~185Ma之间。高压麻粒岩的变质条件、顺时针的P-T轨迹及锆石年代学的结果指示了晚三叠世-早侏罗世的碰撞造山事件(ca.200~166Ma)。结合区域地质资料,推断在西昆仑山内存在一条中生代的中-高压变质带,这条变质带代表了古特提斯洋关闭塔里木与羌塘地块碰撞拼合的位置。     

东昆仑早古生代变质演化:来自浪木日石榴斜长角闪岩的记录SCIEI北大核心CSCD

东昆仑造山带近年来被厘定为早古生代高压-超高压变质带。带内广泛出露早古生代的中-高级变质基性岩,这些岩石记录了不同的变质温压和多期的变质年龄,是反演和制约东昆仑早古生代变质演化的重要样品。本文选取东昆仑浪木日地区的石榴斜长角闪岩为研究对象,开展了变质岩石学及锆石年代学研究。石榴斜长角闪岩呈团块状出露在黑云二长片麻岩中,主要组成矿物为石榴子石、角闪石、斜长石、透辉石和石英,含少量黑云母、绿泥石、金红石、钛铁矿和榍石。石榴子石变斑晶的核部含有绿帘石、角闪石、斜长石、金红石和石英包裹体,其成分从核部到边部,锰铝榴石逐渐降低、钙铝榴石和Mg/(Mg+Fe;)比值逐渐升高,为进变质作用形成的环带。岩石中的矿物结构关系和成分特征显示其经历了进变质、峰期变质和退变质三个阶段的变质演化,变质温压分别为:T≈610℃和P≈6.5kbar、T≈700℃和P≈10.5kbar以及T≈650℃和P≈4.5kbar。这三阶段的变质作用构成顺时针的变质P-T轨迹,指示岩石经历进变质升温升压至峰期阶段,随后经历近等温降压的退变质阶段。同时该P-T轨迹特征表明岩石形成于俯冲-碰撞的构造背景。对石榴斜长角闪岩中的锆石进行SIMS U-Pb定年,得到492.8±5.1Ma的谐和年龄。锆石的形态特征与典型的变质锆石一致,其内包裹的石榴子石、角闪石和斜长石组合与岩石的峰期矿物组合一致。因此,锆石在峰期变质阶段结晶,所测年龄~493Ma为角闪岩相峰期变质年龄。本文研究的石榴斜长角闪岩与该区高压-超高压榴辉岩在野外产状、P-T轨迹和变质年龄等方面密切相关,暗示ca.490Ma是该区高压-超高压变质作用的一个重要时间节点。石榴斜长角闪岩和榴辉岩之间的变质差异,表明东昆仑早古生代经历了多阶段的变质作用,不同岩石记录了原特提斯洋俯冲-碰撞过程的不同阶段。本文获得的变质P-T轨迹和变质年龄可为进一步探究东昆仑早古生代高压-超高压变质作用提供限定。     

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.     

拉萨地块林芝杂岩体石榴角闪岩的地球化学特征和年代学研究

本文对位于青藏高原拉萨地体东南缘林芝杂岩中的两类石榴角闪岩进行了详细的地球化学和锆石U-Pb年代学研究。这两类石榴角闪岩分别为硅不饱和的含十字石石榴角闪岩和硅饱和的含石英石榴角闪岩。含十字石石榴角闪岩矿物组合为十字石、石榴子石、角闪石、钠云母、绿泥石、斜长石。石英石榴角闪岩矿物组合为石榴子石、角闪石、石英、斜长石、黑云母。岩石学及变质相平衡研究表明两类石榴角闪岩均经历了高压角闪岩相变质作用。含十字石石榴角闪岩和石英石榴角闪岩具有MORB的地球化学特征,锆石U-Pb年代学分析获得了800~200 Ma的206Pb/238U年龄范围,出现了~430 Ma、~268 Ma和~216 Ma年龄峰值。~430 Ma年龄可能和拉萨地体岩浆活动有关,~268 Ma变质年龄和~216 Ma变质年龄和拉萨地块经历的高压变质作用有关。其中~268 Ma年龄和拉萨地块内部松多高压带榴辉岩的峰期变质年龄一致,而~216 Ma年龄和榴辉岩的围岩含石榴子石片岩年龄一致。对比该区域的年代学研究成果,这表明林芝杂岩体不仅经历了中新生代的变质和岩浆再造活动,还经历了古特提斯洋闭合,南北拉萨地块发生碰撞的晚二叠世的高压变质作用和三叠纪的中压变质作用。     

Differential response of U-Pb systems in coexisting accessory minerals,Winnipeg River Subprovince,Canadian Shield: implications for Archean crustal growth and stabilization

The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotopic composition of K-feldspar have been investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain.The new data together with published results define the following stages in the evolution of the WRS: (1) an extended period of early crustal growth punctuated by the episodic generation of tonalite. New ages include 3170+20/s-5 Ma, 2875+20/s-5 Ma and 2840+20/s-5 Ma for tonalitic gneisses at Cedar Lake, Kenora and Daniels Lake, respectively. (2) This early evolution was concluded by about 2760 Ma after emplacement of tonalite-granodiorite at Cliff Lake and was followed by a period of magmatic quiescence between about 2760 and 2710 Ma that contrasts with the intensive igneous activity characterizing the evolution of neighbouring greenstone belts. (3) A major episode of magmatism, deformation and metamorphism affected the Kenora and Daniels Lake areas between about 2710 and 2700 Ma. (4) A younger event caused deformation, metasomatism and amphibolite to granulite grade metamorphism at Cedar Lake and Daniels Lake at about 2680 Ma. (5) A subsequent, protracted period of low grade activity reset or (re-)crystallized titanite and apatite defining ages that scatter between about 2640 and 2520 Ma at Cedar and Daniels Lake but not in Kenora where titanite closed by about 2690 Ma. The 2680 Ma metamorphism may have been triggered in part by crustal thickening due to nappe thrusting but the subsequent period of lower grade activity requires the protracted addition of heat and/or fluids probably derived from magmatic and metamorphic processes continuing deep in the crust.The isotopic compositions of K-feldspars are relatively homogeneous and indicate mixing of Pb evolved in different reservoirs. The general enrichment in ²⁰⁷Pb with respect to normal terrestrial Pb reflects the protracted Archean evolution of the terrain.Now-coexisting minerals were formed and closed isotopically at different stages of the complex evolution and were selectively involved or excluded from isotopic equilibration with each other or with external systems such as hydrothermal fluids. This cautions against the indiscriminate interpretation of isotopic values obtained from whole rock systems in such complex terrains.     

大别山北部超高压变质大理岩及其地质意义

岩石学研究表明 ,大别山北部镁铁 超镁铁质岩带中白云质大理岩至少经历过三期变质阶段 :(1)榴辉岩相峰期变质阶段 ,矿物组合主要为方解石 +白云石 +金红石 +镁橄榄石 +钛 斜硅镁石 +富镁的钛铁矿±文石±石榴子石 ;(2 )麻粒岩相退变质阶段 ,矿物组合主要为方解石 +白云石 +金云母 +镁橄榄石 +透辉石 +钛铁矿 +尖晶石±斜方辉石等 ;(3)角闪岩相退变质阶段 ,主要矿物组合为方解石 +白云石 +磷灰石 +磁铁矿+榍石等。它的峰期变质矿物组合 ,类似于苏 鲁超高压大理岩 ,形成压力至少大于 2 .5GPa。这进一步证明 ,大别山北部大多数高级变质岩 (包括大理岩等 )都曾经过超高压变质作用 ,应属于印支期扬子俯冲陆壳的一部分。     

160 Ma of magmatic/hydrothermal and metamorphic activity in the Gällivare area: Re–Os dating of molybdenite and U–Pb dating of titanite from the Aitik Cu–Au–Ag deposit, northern Sweden

Host rocks to the Aitik Cu–Au–Ag deposit in northern Sweden are strongly altered and deformed Early Proterozoic mica(-amphibole) schists and gneisses. The deposit is characterised by numerous mineralisation styles, vein and alteration types. Four samples were selected for Re–Os molybdenite dating and 12 samples for U–Pb titanite dating in order to elucidate the magmatic/hydrothermal and metamorphic history following primary ore deposition in the Aitik Cu–Au–Ag deposit. Samples represent dyke, vein and alteration assemblages from the ore zone, hanging wall and footwall to the deposit. Re–Os dating of molybdenite from deformed barite and quartz veins yielded ages of 1,876±10 Ma and 1,848±8 Ma, respectively. A deformed pegmatite dyke yielded a Re–Os age of 1,848±6 Ma, and an undeformed pegmatite dyke an age of 1,728±7 Ma. U–Pb dating of titanite from a diversity of alteration mineral associations defines a range in ages between 1,750 and 1,805 Ma with a peak at ca. 1,780 Ma. The ages obtained, together with previous data, bracket a 160-Ma (1,890–1,730 Ma) time span encompassing several generations of magmatism, prograde to peak metamorphism, and post-peak cooling; events resulting in the redistribution and addition of metals to the deposit. This multi-stage evolution of the Aitik ore body suggests that the deposit was affected by several thermal events that ultimately produced a complex ore body. The Re–Os and U–Pb ages correlate well with published regional Re–Os and U–Pb age clusters, which have been tied to major magmatic, hydrothermal, and metamorphic events. Primary ore deposition at ca. 1,890 Ma in connection with intrusion of Haparanda granitoids was followed by at least four subsequent episodes of metamorphism and magmatism. Early metamorphism at 1,888–1,872 Ma overlapping with Haparanda (1,890–1,880 Ma) and Perthite-monzonite (1,880–1,870 Ma) magmatism clearly affected the Aitik area, as well as late metamorphism and Lina magmatism at 1,810–1,774 Ma and TIB1 magmatism at 1,800 Ma. The 1,848 Ma Re–Os ages obtained from molybdenite in a quartz vein and pegmatite dyke suggests that the 1,850 Ma magmatism recorded in parts of northern Norrbotten also affected the Aitik area.     

内蒙古锡林浩特岩群岩石学特征及变质温压条件

锡林浩特岩群出露于内蒙古锡林浩特市东南部,指原锡林郭勒杂岩中表壳岩部分,为一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等的变质岩组合。选取岩群中片麻岩及斜长角闪岩进行岩石学及岩相学分析,其中斜长角闪岩主要矿物组合为角闪石+斜长石;片麻岩样品中见夕线石+钾长石矿物组合,石榴子石具明显进变质环带,所以在计算其形成条件时选取了生长于峰期变质阶段的特定部位。运用角闪石斜长石、石榴子石黑云母矿物温压计分别估算锡林浩特岩群中准同时形成的变质基性火山岩及变质碎屑岩的变质温压区间。综合二者计算结果,得锡林浩特岩群峰期变质温压条件为660~707 ℃,0.5~0.6 GPa。变质达高角闪岩相。其变质年龄为1 000 Ma左右,推测锡林浩特岩群角闪岩相变质为中元古代末期锡林浩特微陆块与其他陆块碰撞的结果。     

U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.     

华北克拉通胶北地体多期高级变质事件:来自石榴斜长角闪岩与花岗质浅色体锆石U-Pb定年与稀土元素的新证据

胶北是华北克拉通一个记录了多期高级变质事件的前寒武纪变质地体,确定其每期变质-深熔事件时代与性质对进一步认识胶北乃至华北克拉通早前寒武纪地质演化具有十分重要的科学意义.本文以栖霞石榴斜长角闪岩及其伴生的花岗质浅色体为研究对象,通过锆石内部矿物包体、CL图像、LA-ICP-MS U-Pb定年与稀土元素组成的综合研究,发现石榴斜长角闪岩（19LR53-1）中锆石可进一步划分为两类:第一类（新太古代晚期）锆石通常具有浑圆状晶形且相对弱的（灰黑色）的阴极发光效应,18个该类锆石微区记录了十分一致的207Pb/206Pb年龄,变化于2 540±58 Ma至2 439±54 Ma之间,相应的加权平均年龄为2 498±25 Ma,应代表石榴斜长角闪岩遭受新太古代晚期变质作用的时代;第二类锆石通常具有柱状晶形且相对强的（灰色-灰白色）的阴极发光效应,14个该类锆石微区记录了十分一致的207Pb/206Pb年龄,变化于1 906±54 Ma至1 821±60 Ma之间,相应的加权平均年龄为1 865±30 Ma,被解释为石榴斜长角闪岩遭受古元古代晚期退变质作用的时代.花岗质浅色脉体（19LR53-2）中绝大多数锆石具有不规则状晶形且相对弱的（灰黑色）阴极发光效应,有的含有少量长英质矿物包体,如钾长石+斜长石+石英+磷灰石,18个该类锆石微区记录的207Pb/206Pb年龄变化于2 521±48 Ma至2 453±42 Ma之间,相应的加权平均年龄为2 480±22 Ma,应代表胶北地体新太古代晚期地壳深熔作用的时代.综合本文与前人发表的同位素年代学资料可知,胶北地体太古宙变质基底岩石不仅经历了新太古代晚期~2 500 Ma高级变质作用与地壳深熔作用,而且还叠加了古元古代晚期~1 850 Ma的高级变质作用与地壳深熔作用,它们可能是华北克拉通两次克拉通化过程中的地质响应.      

浙西南八都杂岩早中生代泥质麻粒岩变质作用及构造意义

遂昌-大柘泥质麻粒岩出露于华夏地块东北部的浙西南八都杂岩中,该岩石保留了典型的减压反应结构.但其变质演化特点、变质作用时代及构造意义目前尚不明确.通过系统的岩相学、矿物化学和同位素年代学分析,结果表明遂昌-大柘泥质麻粒岩记录了4个阶段的变质矿物组合,其中早期进变质阶段M₁的矿物组合为石榴石+黑云母+石英;压力峰期变质阶段M₂的矿物组合为石榴石+铝绿泥石+金红石+蓝晶石+刚玉+黑云母+石英±十字石,该矿物组合可能预示着岩石曾经历了超高压变质作用过程;峰期变质阶段M₃的矿物组合为石榴石+黑云母+夕线石+石英±钾长石±斜长石±钛铁矿;峰后近等温降压M_4-1阶段的矿物组合为石榴石+黑云母+夕线石+堇青石+石英+钛铁矿±尖晶石±斜长石±钾长石;M_4-2阶段的矿物组合为石榴石+堇青石+夕线石+斜长石+黑云母+石英±钾长石.相平衡模拟结合传统地质温压计限定其峰期变质阶段的温压条件为T=780~810 ℃、P=8.0~9.2 kbar;峰期后近等温降压的M_4-1阶段的温压条件为T=780~860 ℃和P=5.7~6.0 kbar,M_4-2阶段的温压条件为T=~700 ℃和P=~4.4 kbar,具有典型的顺时针近等温减压型P-T轨迹特征.LA-ICP-MS U-Pb定年结果表明其麻粒岩相变质作用时代为233.5~238.9 Ma.变质作用历史说明浙西南地体可能卷入了古特提斯洋域内印支-华南-华北板块之间的俯冲-碰撞过程,并经历了早中生代的麻粒岩相变质作用后快速折返至地表.      

The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

Metamorphic evolution of ultrahigh-temperature Fe- and Al-rich granulites in the south Yenisei Ridge and tectonic implications

This study provides the first evidence for the occurrence of ultrahigh-temperature (UHT) granulite-facies metamorphism in the Yenisei Ridge (Angara–Kan block). UHT metamorphism is documented in Fe-Al-rich metapelites on the basis of the garnet–hypersthene–sillimanite–cordierite–plagioclase–biotite–spinel–quartz–K-feldspar assemblage. Microtextural relationships and compositional data for paragneisses of the Kan complex attest to three distinct metamorphic episodes: (M1) pre-peak prograde (820?900°C/5.5–7 kbar), (M2) peak UHT (920–1000°C/7–9 kbar), and (M3) post-peak retrograde (770?900°C/5.5–7.5 kbar). The observed counterclockwise P–T evolution at a high geothermal gradient (dT/dP = 100–200°C/kbar) suggests that UHT metamorphic assemblages were formed in an overall extensional tectonic setting accompanied by underplating of mantle-derived mafic magmas, which may be sourced from ~1750 Ma giant radiating dike swarms linked to the Vilyuy mantle plume as part of the Trans-Siberian LIP. The broad synchroneity of UHT metamorphism (1744 ± 26 Ma; monazite–zircon isochron age) and rift-related endogenic activity in the region can provide an additional line of evidence for the two-stage evolution of granulite-facies metamorphism in the Angara–Kan block. The Aldan–Stanovoy, Anabar, and Baikal basement inliers of high-grade metamorphic rocks within the Siberian craton record two Paleoproterozoic peaks (1.9 and 1.75 Ga) of granulite-facies metamorphism. The synchronous sequence of tectonothermal events at the periphery of the large Precambrian Laurentian, Baltica, and Siberian cratons provide convincing evidence for their spatial proximity over a wide time interval, which is consistent with the most recent paleomagnetic reconstructions of the Proterozoic supercontinent Nuna.