Polymetamorphic evolution of the Lewisian complex,NW Scotland,as recorded by U-Pb isotopic compositions of zircon,titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of 2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at 1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.     

Geochronological framework of the Quadrilátero Ferrífero,with emphasis on the age of gold mineralization hosted in Archean greenstone belts

Archean terrains of the Quadrilátero Ferrífero comprise a greenstone belt association surrounded by granitoid–gneiss complexes, mainly composed of banded TTG gneisses whose igneous protoliths are older than 2900 Ma. This early continental crust was affected by three granitic magmatic episodes during the Neoarchean: ca. 2780 to 2760 Ma; 2720 to 2700 Ma; and 2600 Ma. Dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 Ma, contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the Quadrilátero Ferrífero. Lead isotopic studies of lode–gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 Ma.Proterozoic evolution of the Quadrilátero Ferrífero comprises deposition of a continental-margin succession hosting thick, Lake Superior-type banded iron formations, at ca. 2500 to 2400 Ma, followed by deposition of syn-orogenic successions after 2120 Ma. The latter is related to the Transamazonian Orogeny. The western part of the Quadrilátero Ferrífero was also affected by the Brasiliano Orogeny (600 to 560 Ma).     

Unscrambling the stratigraphy of an Archean greenstone belt; a U---Pb geochronological study of the Favourable Lake belt, northwestern Ontario, Canada

New zircon and baddeleyite U---Pb ages show that the major units of the Archean Favourable Lake greenstone belt in the northern Superior Province were formed by episodic magmatism spanning more than 250 Ma and demonstrate that the original stratigraphy is disrupted by thrusts that juxtaposed older supracrustal rocks on top of younger ones. The oldest rocks of the area are a 2950 Ma gneissic tonalite and a 3000-2960 Ma granodiorite clast from a ca. 2725 Ma conglomerate. Five distinct volcanic (and sedimentary) groups formed during presumably short-lived episodes at 2925 Ma (I), 2870 Ma (II), 2858 Ma (III), 2734 Ma (IV), and 2725 Ma (V). The youngest group contains the thickest sedimentary unit, a turbiditic and alluvial-fluvial sequence. Compression caused thrusting that placed Groups I and V on top of IV, III on V, and II on III. The thrust complex was subsequently isoclinally folded. Compression was accompanied by major plutonism that emplaced the bulk of the bounding batholiths between 2732 and 2711 Ma ago. The late tectonic, Mo-mineralized Setting Net Lake Stock in the centre of the belt has an age of about 2708 Ma and was overprinted by younger hydrothermal events that produced monazite (2706 Ma), titanite (ca. 2695-2690 Ma) and rutile (the youngest rutile at 2657 Ma). Similar late hydrothermal pulses are recorded by secondary titanite elsewhere in the belt and within the batholiths. The protracted magmatic evolution of the belt is typical of that observed in a number of greenstone belts of the northern Superior Province but is uncommon farther to the south. In contrast, the structural complications and put-of-sequence stratigraphy appear to be a quite common tectonic characteristic of greenstone belts of the whole Superior Province.     

鲁东地区侵入岩形成时代和期次划分——锆石SHRIMPU—Pb年龄的证据

鲁东地区前寒武纪侵入岩其侵入活动集中于新太古代和新元古代,均遭受不同程度的区域变质和韧性剪切带的改造,形成一套花岗质片麻岩类。新太古代早期TTG质花岗岩分布于胶北隆起区莱州一栖霞一带,经受了区域变质作用,形成一套灰色片麻岩,锆石内核SHRIMPU—Pb年龄2726—2707Ma基本代表岩浆结晶年龄,锆石变质边为2500Ma;古元古代侵入岩——莱州基性一超基性岩组合形成于1900～1850Ma左右,锆石SHRIMPU—Pb年龄（1852±9）Ma,（1868±11）Ma;新元古代侵入岩主要为花岗闪长质-二长花岗质片麻岩,出露于胶南-威海造山带,锆石U～Pb定年结果集中为892～605Ma,其中荣成超单元锆石SHRIMPU—Pb年龄720～780Ma。中生代侵入岩以花岗岩类为主,集中出露于半岛北部和东南沿海一带。燕山早期侵入岩文登超单元、垛崮山超单元、玲珑超单元锆石SHRIMPU—Pb同位素年龄值集中在167～150Ma;燕山晚期侵入岩大店超单元锆石SHRIMPU～Pb年龄120±4Ma,崂山超单元锆石SHRIMPU～Pb同位素年龄值为（120±2）～（114±2）Ma。     

丹凤地区秦岭岩群片麻岩锆石U-Pb年龄:北秦岭地体中-新元古代岩浆作用和早古生代变质作用的记录

秦岭岩群被认为是出露于北秦岭地体内最古老的前寒武纪基底岩石,记录了北秦岭造山带的地壳形成和演化历史。本文报道丹凤-西峡地区五件秦岭岩群片麻岩锆石U-Pb年龄结果,限定其形成和变质时代,探讨北秦岭地体的构造归属。定年结果表明,岩浆成因锆石颗粒的年龄集中在1400～1600Ma左右和850～950Ma左右,记录两期主要岩浆活动。6粒锆石具有变质成因特征,低Th/U比值(0.03),206Pb/238U年龄变化在510～465Ma之间,加权平均值477±18Ma。这一古生代变质叠加时代与北秦岭地体南北缘高压变质作用时代基本一致,说明秦岭岩群遭受到北秦岭造山带俯冲-碰撞造山过程的变质作用。秦岭岩群主要形成于中元古代晚期至新元古代早期,基底岩石缺乏早元古代和太古代岩浆活动的记录。在岩浆作用时代上,北秦岭地体与广泛发育新元古代中-晚期岩浆作用的扬子陆块北缘有差别,也不同于晚太古代-早元古代的华北陆块南缘,可能是中-新元古代形成的独立微陆块。     

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.     

Geochronological framework of the Quadrilátero Ferrífero, with emphasis on the age of gold mineralization hosted in Archean greenstone belts

Archean terrains of the Quadrilátero Ferrífero comprise a greenstone belt association surrounded by granitoid–gneiss complexes, mainly composed of banded TTG gneisses whose igneous protoliths are older than 2900 Ma. This early continental crust was affected by three granitic magmatic episodes during the Neoarchean: ca. 2780 to 2760 Ma; 2720 to 2700 Ma; and 2600 Ma. Dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 Ma, contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the Quadrilátero Ferrífero. Lead isotopic studies of lode–gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 Ma.Proterozoic evolution of the Quadrilátero Ferrífero comprises deposition of a continental-margin succession hosting thick, Lake Superior-type banded iron formations, at ca. 2500 to 2400 Ma, followed by deposition of syn-orogenic successions after 2120 Ma. The latter is related to the Transamazonian Orogeny. The western part of the Quadrilátero Ferrífero was also affected by the Brasiliano Orogeny (600 to 560 Ma).     

A newly defined Late Ordovician magmatic‐thermal event in the Mt Painter Province,northern Flinders Ranges,South Australia

Magmatism,metamorphism and metasomatism in the Palaeoproterozoic‐Mesoproterozoic Mt Painter Inlier and overlying Neoproterozoic Adelaidean rocks in the northern Flinders Ranges (South Australia) have previously been interpreted as resulting from the ca 500 Ma Delamerian Orogeny. New Rb–Sr, Sm–Nd and U–Pb data, as well as structural analysis,indicate that the area also experienced a second thermal event in the Late Ordovician (ca 440 Ma). The Delamerian Orogeny resulted in large‐scale folding, prograde metamorphism and minor magmatic activity in the form of a small volume of pegmatites and leucogranites. The Late Ordovician event produced larger volumes of granite (the British Empire Granite in the core of the inlier) and these show Nd isotopic evidence for a mantle component. The high‐temperature stage of this magmatic‐hydrothermal event also gave rise to unusual diopside‐titanite veins and the primary uranium mineralisation in the basement, of which the remobilisation was younger than 3.5 Ma. It is possible that parts of the Mt Gee quartz‐hematite epithermal system developed during the waning stages of the Late Ordovician event. We suggest that the Ordovician hydrothermal system was also the cause of the commonly observed retrogression of Delamerian metamorphic minerals (cordierite, andalusite) and the widespread development of actinolite, scapolite, tremolite and magnetite in the cover sequences. Deformation during the Late Ordovician was brittle. The recognition of the Late Ordovician magmatic‐hydrothermal event in the Mt Painter Province might help to link the tectonic evolution of central Australia and the southeast Australian Lachlan Fold Belt.     

Timing of high-grade metamorphism: Early Palaeozoic U–Pb formation ages of titanite indicate long-standing high-T conditions at the western margin of Gondwana (Argentina, 26–29°S)

Abstract Concordant U–Pb ages of c. 530–510 Ma and c. 470–420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high‐T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26–29°S). The U–Pb ages document the time of titanite formation at high‐T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were > c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U–Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U–Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U–Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation‐enhanced recrystallization of titanite and/or different thermal peaks during a long‐standing, geographically fixed, high‐T regime in the mid‐crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region.     

Timescales and significance of high-pressure, high-temperature metamorphism and mafic dike anatexis, Snowbird tectonic zone, Canada

New geochronological, isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1.9 Ga high pressure, high temperature metamorphism along the Snowbird tectonic zone in northern Saskatchewan. High-precision U–Pb zircon dates reveal anatexis of Chipman mafic dikes at 1,896.2 ± 0.3 Ma during syntectonic and synmetamorphic intrusion at conditions of 1.0–1.2 GPa, >750°C. U–Pb zircon dates of 1,894–1,891 Ma for cross-cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels. The persistence of elevated temperatures for ~14 m.y. following peak conditions is implied by younger U–Pb titanite dates, and by Sm–Nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source. Limited melting of the host felsic gneiss at 1.9 Ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the Archean. Spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability. We correlate 1,896 Ma Chipman mafic dike emplacement and metamorphism with substantial 1.9 Ga mafic magmatism over a minimum along-strike extent of 1,200 km of the Snowbird tectonic zone. This suggests a significant, continent-wide period of asthenospheric upwelling that induced incipient continental rifting. Extension was subsequently terminated by hinterland contraction associated with Trans-Hudson accretion and orogenesis. Little activity in the lower crust for ca. 650 m.y. prior to Proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1.9 Ga. This episode of destabilization contrasts with the record of long-term stability in most preserved cratons, and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents.     

U–Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland

Abstract A metasomatic diopside rock occurs at the top of the dolomitic Connemara Marble Formation of western Ireland and contains titanite and K-feldspar in addition to around 90% diopside ( X _Mg= 0.90–0.97). U–Pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium-rich and isotopically concordant, with the result that a precise U–Pb age of 478 ± 2.5 Ma can be determined. The age is identical within error to a less precise Rb–Sr age of diopside–K-feldspar of 483 ± 6 Ma. Petrological evidence indicates that the assemblage crystallized at c . 620° C close to or below the closure temperature of titanite. The age thus provides a precise estimate of the time of metamorphism; this age is 11 ± 3 Ma younger than the 490 Ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age. This difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara.     

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.     

辽东半岛～2．2 Ga 岩浆事件及其地质意义

本文报道了辽东半岛古元古代胶—辽—吉活动带内辽河群变质火山岩和辽吉花岗岩的锆石LA-ICP-MS U-Pb年代学和地球化学数据。变质安山岩的锆石具典型的岩浆振荡环带结构和较高的Th/U值(0.5),锆石U-Pb年龄为(2 182±6)Ma和(2 229±22)Ma,该年龄可代表安山岩的形成年龄。辽吉花岗岩的锆石同样具有典型的岩浆振荡环带结构和较高的Th/U值(0.3),锆石U-Pb年龄为(2 199±10)Ma,代表花岗岩的侵位时代,在误差范围内与辽河群火山岩喷发时代一致,表明辽吉花岗岩并不是辽河群的基底,二者可能为同一次岩浆作用过程的产物。辽东半岛~2.2Ga岩浆事件的识别及性质,对于正确认识古元古代胶—辽—吉活动带的属性至关重要。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶—辽—吉活动带的形成演化可能与弧-陆碰撞有关。     

U-Pb monazite, xenotime and titanite geochronological constraints on the prograde to post-peak metamorphic thermal history of Paleoproterozoic migmatites from the Grand Canyon, Arizona

The petrology and U-Pb geochronology of pelitic migmatite and calc-silicate gneiss reveal a detailed prograde to post-peak metamorphic thermal history for a single outcrop of Paleoproterozoic supracrustal rocks in the eastern part of the Grand Canyon. Metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 Ma and continued to grow until about 1697 Ma. The U-Pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting, which involved the breakdown of muscovite to sillimanite, commenced at about 1702 Ma, prior to the metamorphic peak. Partial melting continued until about 1690 Ma, the youngest U-Pb date from magmatic monazite in the leucosomes. Field and petrographic evidence, as well as inheritance patterns in monazites from the leucosomes, suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region. Between 1702 and 1690 Ma, the migmatite package heated to peak metamorphic conditions of about 720 °C and 6 kbar, cooled to about 675 °C at a cooling rate >30 °C/million years, and decompressed to about 4 kbar. The U-Pb geochronological data for metamorphic titanite from a calc-silicate gneiss exhibit a clear relationship between grain size and the ²⁰⁷Pb/²⁰⁶Pb date indicating that the titanite crystals record cooling ages. These data, combined with the titanite Pb diffusion data of Cherniak (1993), yield a cooling rate of 5.4_−0.9 ^+1.7 °C/million years, integrated over the interval 1690 to 1676 Ma and suggest that by 1675 Ma, the cooling rate slowed to less than 2 °C/million years. The rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large-scale tectonic processes associated with the accretion of juvenile crust to the margin of Laurentia. Juvenile arc crust appears to have been assembled, accreted and stabilized into Laurentian lithosphere in less than 30 million years. Received: 21 January 1998 / Accepted: 27 August 1998     

Protolith age of eclogites from the southern part of Pezhostrov Island,Belomorian belt: Protolith of metabasites as indicator of eclogitization time

Several types of metabasites of different age were identified in the southern part of Pezhostrov Island: eclogites with a magmatic protolith age of about 2200 Ma and 2500 Ma old metagabbroanorthosites that retained no eclogitic assemblage. It is shown that the Paleoproterozoic eclogites dominate volumetrically over Archean eclogites in the Belomorian eclogitic province. Eclogites with the youngest Jatulian protolith age (no older than 2200 Ma) occur with the same frequency as those with the Archean protolith age. A new find of eclogites with a Paleoproterozoic age of magmatic protolith and generalization of accumulated geochronological data confirm the recognition of an extended zone of high-pressure metamorphism with an age around 1900 Ma in the Belomorian mobile belt.     

The age and source of late Hercynian magmatism in the central Alps: evidence from precise U−Pb ages and initial Hf isotopes

This study presents U–Pb ages for zircon, titanite, allanite and epidote, and initial Hf isotopic compositions for zircon of Upper Carboniferous granites, diorites and syenites from the Aar massif, central Alps. The rocks were emplaced during three magmatic pulses after Hercynian collisional tectonics: (A) a shoshonitic-ultrapotassic series at 334±2.5 Ma; (B) scattered diorites and granites at 308–310 Ma; and (C) a high-K cale-alkaline granite batholith at 298±2 Ma. Inheritance of old zircons is negligible among all three groups. The Southern Aar granite, in contrast, is a syn-tectonic, probably ca. 350 Ma old granite that contains large amounts of inherited Precambrian zircons. Alpine metamorphism caused weak lead loss in many analyzed zircon fractions, but left the titanite U–Pb system undisturbed: thorites were almost completely reset by Alpine and recent lead loss. Mineral isochrons defined by titanite, allanite, epidote and apatite yield initial Pb isotopic compositions that are in agreement with the model values of Stacey and Kramers. Initial Hf isotopic compositions range from _Hf=–8 to +3.5. The data follow a trend of increasing _Hf with decreasing age. The _Hf versus element concentration relationships suggest mixing between a mantle and a crustal component. These relationships can be explained in terms of generation of the melts from a subcontinental mantle that had been enriched during subduction events at about 1 Ga and by 300 Ma had developed an isotopic signature distinct from that of MORB-type mantle. Further contamination of the melts occurred during ascent and differentiation in the crust. This late Hercynian magmatism can be related to post-collisional strike-slip tectonics.     

Stability of Al- and F-rich titanite in metacarbonate: petrologic and isotopic constraints from a polymetamorphic eclogitic marble of the internal Sesia Zone (Western Alps)

Al- and F-rich titanite (3.862O₃ <9.33 wt%, 0.931(F, OH)₁Ti_-1O_-1 substitution within single crystals, particularly at grain boundaries with omphacite and/or phengite. In-situ ion microprobe U-Pb analysis of titanite domains that have various Al and F contents yielded apparent ²⁰⁶Pb/²³⁸U ages scattering between 283 and 153 Ma. Chemical and petrological data are indispensable to interpret this complex age distribution, and the good correlation between ²⁰⁶Pb/²³⁸U ratios and Al content indicates that the Al- and F-rich titanite was formed during pre-Alpine metamorphism (𔕑ᆟ Ma). Progressively younger ages are obtained in domains with decreasing Al and F content, suggesting that partial chemical re-equilibration was responsible for the incomplete isotopic resetting during Alpine metamorphism. Petrological and U-Pb data show that Al- and F-rich titanite should be used with caution to infer high-pressure conditions in polymetamorphic carbonate systems.     

U-Pb systematics of garnet: dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes,Manitoba, Canada

This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured ²⁰⁶Pb/²⁰⁴Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce ²⁰⁷Pb/²⁰⁶Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.     

New age constraints on metamorphism,metasomatism and gold mineralisation at Plutonic Gold Mine,Marymia Inlier,Western Australia

The Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb–Sr ages for mica, as well as a new ²⁰⁷Pb–²⁰⁶Pb age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300–2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb–Sr age of 2296 ± 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb–Sr system in muscovite at about the same time. A younger Rb–Sr age of 1779 ± 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820–1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new ²⁰⁷Pb–²⁰⁶Pb age of 1725 ± 26 Ma for titanite in a chlorite–carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U–Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300–2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730–1660 Ma. The 2300–2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.     

台湾东部未变形花岗岩LA-ICP-MS锆石U-Pb定年及其地质意义

台湾源头山位于台湾岛东部太鲁阁带的北部,主体为花岗闪长岩-辉长岩"浆混岩"组合,发生了强烈变形变质,其中央分布有未变形的侵入体,具有典型岩浆岩特征,代表了变形变质期后的岩浆事件。通过开展野外地质调查和岩石学、同位素年代学研究,揭示了变形变质作用前后侵入岩在岩石特征及主量、微量元素方面存在明显差别,表明二者是不同构造岩浆事件的产物。未变形的侵入岩为石英闪长岩,具有高Pb、Rb、U、Th、Zr轻度富集特征,属典型的Ⅰ型花岗岩,其LA-ICPMS锆石U-Pb年龄为82±1Ma,属晚白垩世坎潘期。这是台湾东部地区变形变质事件结束的标志,其后台湾东部进入了一个非挤压的新构造作用阶段。