Inherited Zircon Ages Research Articles

Ordovician–Silurian rift-passive margin on the Mexican margin of the Rheic Ocean overlain by Carboniferous–Permian periarc rocks: Evidence from the eastern Acatlán Complex, southern Mexico

In the Xayacatlán area of the Acatlán Complex in southern Mexico, several, N–S, fault-bounded packages of rocks occur (from west to east): (i) greenschist facies, interbedded psammites and pelites (Huerta Unit): the youngest concordant detrital zircon in the western part of the unit is 455 ± 4 Ma; (ii) greenschist-amphibolite facies arkoses and psammites (Amate Unit): the youngest concordant detrital zircon gave a 206Pb/ 238U age of 902 ± 14 Ma, and older concordant ages of ~ 900–1300 Ma; (iii) low grade clastic rocks and marbles (Permian Tecomate Formation); and (iv) greenschist facies psammites, pelites and thin tholeiitic mafic slices (Salada Unit): the youngest concordant detrital zircon yielded a 206Pb/ 238U age of 352 ± 3 Ma and older concordant ages of ~ 434–485 Ma, 511–630 Ma, and 920–1200 Ma. The eastern part of the Huerta Unit is cut by pegmatite that yielded an almost concordant 206Pb/ 238U zircon age of 464 ± 4 Ma (a mean age of the four almost concordant young zircons is ~ 485.5 Ma) interpreted as the time of intrusion, and inherited zircons with concordant ages of ~ 530–580 Ma and ~ 910–1200 Ma. These data suggest that the Huerta Unit ranges in age from Early (possibly older) to Late Ordovician. The Amate Unit is cut by granitic dikes, in which the youngest concordant 206Pb/ 238U zircon ages are 452 ± 6 Ma and 447 ± 3 Ma interpreted as the time of intrusion, and older inherited zircon ages with concordant 206Pb/ 238U ages in the range of 950–1250 Ma. The intrusive ages are similar to the 442 ± 1 Ma concordant U–Pb age for mafic dikes that also cut the Amate Unit. These data constrain the depositional age of the Amate Unit between ~ 900 Ma and latest Ordovician. The age of the Salada Unit is probably Carboniferous because it is structurally overlain by the Lower–Middle Permian Tecomate Unit. The preponderance of 510–630 Ma and 900–1250 Ma ages in both the detrital and inherited ages in some of the dated samples suggests a source in the adjacent and subjacent Maya and Oaxacan Complex, respectively, although a xenocrystic host rock source cannot be ruled out for the intrusive rocks. The igneous intrusions have been interpreted as part of a 480–440 Ma bimodal rift-related suite. Their synchroneity with deposition of the Huerta Unit (and possibly also the Amate Unit) suggests a rift-passive margin environment of deposition on the southern margin of the Rheic Ocean. On the other hand, the Salada Unit and the Tecomate Formation appear to be synchronous with a Permo-Triassic magmatic arc that developed on the western margin of Pangea.

The Tectono-Thermal Events of Taiwan and Their Relationship with SE China

We present a new synthesis of the tectono-thermal events of Taiwan, excluding the Coastal Range, based on existing isotopic, geochemical and geochronological data for granitic, metamorphic, volcanic and sedimentary rocks. Nd model ages (TDM) and the inherited zircon ages consistently yielded Proterozoic ages, suggesting that the source rocks from the exposed rocks in Taiwan were formed in the Proterozoic, starting from about 2 Ga ago. The crustal evolution of Taiwan began in the Late Paleozoic (250 ± 20 Ma). Since then, five tectono-thermal events can be delineated: (I) an Early Jurassic event (200 175 Ma) registered in the marble and metapelites of the Tananao metamorphic basement complex of northern Taiwan and crystalline limestone of the basement rocks in western Taiwan; (II) a Late Jurassic event (~153 Ma) revealed by a meta-granite of the Tananao metamorphic basement complex of southern Taiwan; (III) a Late Mesozoic event (97 - 77 Ma) recorded in the rocks of the Tananao metamorphic basement complex and offshore of northern and western Taiwan; (IV) a Cenozoic of prePliocene event (episodic from 56 to 9 Ma) registered in the dikes in the Central Range and the intraplate basalts of mainland Taiwan and offshore of northern and western Taiwan; and (V) an ongoing Late Cenozoic event (since 5 Ma) shown in the recent volcanics of onshore and offshore northern Taiwan and offshore northeastern Taiwan. In comparison with the Cathysia foldbelts, Taiwan demonstrates similar Paleoproterozoic crustal residence ages, coeval Jurassic magmatism (Early Yanshanian) and Late Cretaceous (Late Yanshanian) orogeny. However, subduction-related magmatism was prevalent in Taiwan in comparison with both extension- and subduction-related magmatism in Cathaysia foldbelts during the Yanshanian orogeny. Rifting-related magmatism has continued in the Cathaysia foldbelts ever since Jurassic. Since Late Cretaceous, rifting-related magmatism migrated from the interior toward the coastal region in the Cathaysia foldbelts but it did not reach Taiwan until Paleogene. Since 5 Ma, subduction-related activity started in northern Taiwan. The volcanism in northern Taiwan was attributed to the Late Pliocene extensional collapse of the northern Taiwan mountain belt in response to the oblique collision of the northern Luzon arc with the Asian continent, but not directly related to the magmatism of the Ryukyu subduction zone. However, volcanism resulting from the spreading of the Okinawa trough manifests in the offshore region of northeastern Taiwan.

Chronological and geochemical studies of granite and enclave in Baimashan pluton, Hunan, South China

Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low ɛ Nd(t) values but high (87Sr/86Sr)i ratios, and old T 2DM (ca. 1.9–2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.

Generation of Late Cretaceous silicic rocks in SE China: Age, major element and numerical simulation constraints

Rhyolite-dominating bimodal volcanic suites (rhyolite/basalt), mafic dikes and A-type granites distribute from N Zhejiang to S Fujian over 800 km in the Southeast Coast Magmatic Belt (SCMB) – the Late Yanshanian (LY) orogenic belt in SE China. Data of 40Ar/ 39Ar and K–Ar whole-rock ages and LA-ICPMS U–Pb zircon ages indicate that rhyolitic volcanism (101–72 Ma) is contemporaneous with the A-type granitic intrusions (100–90 Ma) and mafic dike injections (94–77 Ma). This time span is used to define the upper volcanic series in Zhejiang–Fujian areas. One striking feature of rhyolites in the SCMB is that many are strongly peraluminous (SP) and others, mostly restrict in Fujian, are peralkaline to mildly peraluminous (P-MP) and chemically resemble A-type granites. The SP character is unique among well-known large rhyolite provinces worldwide. Based on experimental works for a common thermal regime and inherited zircon age information, we suggest that SP and P-MP rhyolites represent low pressure melting of the felsic (quartzofeldspathic) granite (±metapelite) and the accompanied granodioritic, tonalitic and trondhjemitic member of the core complex assemblage, respectively, to account for the decreasing aluminosity. They could have also been contaminated by young igneous rocks, and ancient crust to a lesser degree, during ascent to the surface. Plate subduction and lithosphere extension processes, respectively, are numerically simulated for the magma generation of these rhyolites using the mafic underplating model. Results suggest that the most effective controlling factor to generate SP and associated P-MP (A-type) magmas during 95–80 Ma is thinning of the lithosphere thickness with a high exhumation rate. Under this circumstance, the core complex assemblage can be uplifted to lower level of the crust and match the constraint of experimental works.

U-Pb SHRIMP zircon dating of andesite from the Dolomite area (NE Italy): geochronological evidence for the early onset of Permian Volcanism in the eastern part of the southern Alps

The Athesian Volcanic District (AVD), a thick sequence of andesitic to rhyolitic lava and ignimbrite, overlies both the Variscan basement of the Dolomites and, where present, the continental basal conglomerate of Upper Carboniferous(?) to Early Permian age. This volcanic activity is known to mark the margin of the intra-Pangea megashear system between Gondwana and Laurasia, the onset age of which is determined in this study. SHRIMP U-Pb dating on zircon from Ponte Gardena/Waidbruck (Isarco/Eisack valley) basaltic andesite yields an age of 290.7 ± 3 Ma, providing the oldest record of andesite volcanic activity yet documented in the AVD. Two younger dates (279.9 ± 3.3 and 278.6 ± 3.1 Ma) obtained for the andesitic necks of M. dei Ginepri (Eores/Aferer valley) and Col Quaternà (western Comelico), respectively, probably represent a second pulse of andesite magmatic activity. Near Chiusa/Klausen, the volcanoclastic deposits at the bottom of the Funes/Villnöss valley volcano-sedimentary complex only contain detrital zircons, dated at 469 ± 6 Ma; these probably derive from erosion of Paleozoic porphyroids. Other zircons from the same sediments and inherited cores of magmatic andesite crystals give Paleoproterozoic (1953.6 ± 22.1, 1834.6 ± 69.3, 1773.6 ± 25.1 Ma), Early Neoproterozoic (1015 ± 14 Ma) and Late Neoproterozoic (728.4 ± 9.6, 687.6 ± 7.6 Ma) ages. These ancient detrital and inherited zircon ages fit the model that envisages the Dolomite region as being tectonically coherent with Africa, at least until the Lower Permian.

Devonian deep-crustal metamorphism and exhumation in the Variscan Orogen: evidence from SHRIMP zircon ages from the HT-HP granulites and migmatites of the Góry Sowie (Polish Sudetes)

The high-temperature and high-pressure granulites in the internal zone of the Variscan belt are witnesses of deep crustal processes and subsequent exhumation of deeper lithospheric fragments. The Góry Sowie Block granulites in SW Poland and the surrounding gneisses and migmatites contain different inherited zircon age spectra, testifying different sources of their protoliths: mainly Cadomian (ca. 580 Ma) igneous rocks, and early Ordovician (ca. 500 Ma) granitoids (or their reworked products), respectively. The metamorphic spheric and oval zircons in the granulites give two statistically distinct ages of c. 395 and 380 Ma, in excess of experimental uncertainty. These ages may correspond to the high-temperature (HT) and high-pressure (HP) granulite facies metamorphism and subsequent amphibolite facies retrogression. However, essentially isochemical reconstitution of zircons and inheritance of radiogenic Pb cannot yet be excluded. The retrogression in the granulites coincided with the amphibolite facies metamorphism in the surrounding gneisses evidenced by a range of various isotopic ages between 384 and 370 Ma. The new SHRIMP zircon data provide new evidence of the presence of “old” (c. 400–395 Ma) HT-HP granulites in the central European part of the Variscides. The granulites were subsequently exhumed to mid-crustal levels, and tectonically interleaved with gneisses. Continuing uplift and decompression caused migmatization in the surrounding gneisses and partial re-equilibration of the granulites, at around 380 Ma. This high-temperature and medium-pressure (HT-MP) event was followed by rapid uplift and exhumation at c. 360 Ma due to Eo-Variscan orogenic movements.

Derivation of Mesozoic adakitic magmas from ancient lower crust in the North China craton

Three Mesozoic plutons were intruded into the Archean granulite terrains in northern margin of the North China craton adjacent to the granulite xenolith-bearing Tertiary Hannuoba basalts. They are Triassic Guzuizi and Honghualiang granites and Early Cretaceous Zhuanzhilian diorite. Rocks of the three intrusions are characterized by high Sr (576–1216 ppm) and Na 2O (⩾4.0%), low Y (⩽18 ppm) and heavy rare-earth elements (Yb < 1.8 ppm). These features are similar to modern adakites (siliceous rocks with high sodium, aluminum and strontium and low heavy rare-earth element and yttrium contents) from island arcs and Archean high-Al tonalite–trondhjemite–granodiorite (TTG). However, they are distinctly potassium-rich and their evolved strontium, neodymium, and hafnium isotopic compositions and inherited zircon ages coincide with those of the Archean granulites and the Hannuoba granulite xenoliths. Such features cannot readily be explained with previous models of the origin of adakites and TTGs. We propose that these adakitic rocks are formed by partial melting of ancient lower crust, and that the restites are represented by some of the Hannuoba granulite xenoliths; we further argue that this could be one major mechanism to generate voluminous Mesozoic adakitic magmas in the eastern North China craton. Our hypothesis can also explain the discrepancies in ages and compositions between the granulite terrains and xenoliths, and implies that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust.

496 ± 3 Ma zircon ion microprobe age for pre-Hercynian granite, Central Iberian Zone, NE Portugal (earlier claimed 618 ± 9 Ma)

Ion microprobe (SIMS) dating of zircon from the Miranda do Douro orthogneiss, Central Iberian Zone, Hercynian Iberian belt, defines an Early Ordovician U–Pb age of 496.0 ± 2.6 Ma (95 % conf., MSWD = 1.14) for magmatic zircon crystallization in its granitic protolith. The age contrasts with an earlier, conventional (ID-TIMS) U–Pb zircon age of 618 ± 9(95 % conf.) Ma, now thought to be an artefact of the complex zircon population. Individual SIMS ages for zircon from the rock range from 2700 to 180 Ma and comprise inherited and magmatic zircon, both concordant and common Pb-enriched, and younger, reset ages. The ID-TIMS study seems to have misinterpreted this heterogeneous population as a cogenetic suite consisting of magmatic zircon and its age-reset equivalents produced by recent Pb loss. The 496 ± 3 Ma SIMS age represents the weighted average for 26 magmatic zircon domains located by careful inspection of cathodoluminescence, secondary electron and optical microscopy images of ∼ 700 individual zircon crystals. Inherited zircon is widespread, ranging in age from 2700 to 550 Ma, with age clusters, which are statistically indistinguishable from those known from pre-Hercynian granitic basement material elsewhere in the Central Iberian Zone. Including the present age information, 582 ± 4 Ma (95 % conf., MSWD = 1.02, n = 13) and 619 ± 9 Ma (95 % conf., MSWD = 0.93, n = 7) appear as prevailing inherited zircon age components in basement intrusions in the Central Iberian Zone.

The Cadomian unconformity in the Saxo-Thuringian Zone, Germany: Palaeogeographic affinities of Ediacaran (terminal Neoproterozoic) and Cambrian strata

The most complete Ediacaran (terminal Neoproterozoic) to Early Cambrian record in the Saxo-Thuringian Zone is provided by the Cadomian-deformed Ediacaran Rothstein Formation and the unconformably overlying Early Cambrian Zwethau Formation in the Torgau–Doberlug Syncline (TDS). Conglomerates and greywackes of the marine Rothstein Formation are of continental magmatic arc provenance and record detrital zircon SHRIMP ages that indicate a Late Cryogenian to Early Ediacaran (700–580 Ma) age for the arc source and its emplacement into Palaeoproterozoic (2000 Ma) crust. Tuffaceous intercalations record extrabasinal eruptions of evolved calc-alkaline lavas that point to ongoing Cadomian continental arc magmatism. SHRIMP zircon ages from a tuffaceous layer date this volcanic activity and the formation's deposition as Late Ediacaran (566 ± 10 Ma). Close correlation in provenance and age to other Ediacaran units of the Saxo-Thuringian Zone points to a common palaeogeographic setting in the Avalonian–Cadomian belt. Detrital and inherited zircon ages, and Nd-isotopic ratios from these Ediacaran siliciclastic rocks suggest a position on the active margin of Gondwana near the West African craton. The Early Cambrian Zwethau Formation records the erosion of the underlying Ediacaran rocks followed by the successive evolution of: (i) a carbonate-dominated subtidal ramp with calcimicrobial–archaeocyathan buildups, (ii) a shallow-subtidal to intertidal mixed ramp with peritidal sediments, and oolite shoal complexes, and (iii) a more siliciclastic depositional environment. Calcimicrobial–archaeocyathan buildups, oolite shoals, and sulfate nodules in intertidal sediments indicate warm, arid to semi-arid climatic conditions and a palaeogeographic setting in equatorial to sub-equatorial latitudes. Archaeocyaths, the trilobite taxon Dolerolichia, and the sedimentary facies assemblages compare closely to those of the “Mediterranean” Early Cambrian, constraining the palaeogeographic setting to the European, sub-equatorial, western Gondwana shelf realm. Archaeocyaths record a mid-Early Cambrian (Middle Issendalenian/Ovetian) age, making these the oldest Cambrian sediments in both the Saxo-Thuringian Zone and the Bohemian Massif. The Middle Cambrian lithological and palaeontological records of the TDS closely resemble those of the Frankenwald area of the Saxo-Thuringian Zone as well as those of other fragments of the western Gondwana shelf in Spain, Morocco, and Bohemia. Archaeocyaths, Early Cambrian trilobites, and correlations with Morocco suggest Cambrian deposition commenced at ca. 520 Ma, such that the Cadomian unconformity represents a time gap of about 35–55 Ma (latest Ediacaran to Early Lower Cambrian). This gap, which is related to the Cadomian orogeny and so common to all areas of the Cadomian belt, obscures some of palaeobiological, palaeogeographic, and climatic change that marks the Precambrian–Cambrian transition.

Further evidence for ∼1.85 Ga metamorphism in the Central Zone of the North China Craton: SHRIMP U–Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province

This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua ‘Group’ gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84 ± 0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua ‘Group’, gives a magmatic zircon age of 2.14 ± 0.02 Ga and a metamorphic zircon age of ∼1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua ‘Group’ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.

Surgeon island granite SHRIMP zircon ages: a clue for the Cambrian tectonic setting and evolution of the Palaeopacific margin of Gondwana (northern Victoria Land, Antarctica)

AbstractIn northern Victoria Land (NVL), Antarctica, the palaeopacific margin of Gondwana is made up of the inboard Wilson (WT) and the two outboard Bowers (BT) and Robertson Bay (RBT) terranes. The occurrence of a Cambro‐Ordovician magmatic arc in the WT argues for a southwestward subduction leading to the final configuration of this margin during the Ross–Delamerian Orogeny. A U‐Pb SHRIMP crystallization age (511.7 ± 2.9 Ma) obtained on the Surgeon Island Granite (SIG), located at the eastern end of the RBT, indicates that the SIG also belongs to the Ross cycle, and provides evidence for multiple subduction zones during the Ross Orogeny. Structural observations show that the SIG and its country rocks are basement to the RBT turbidites. SIG inherited zircon ages indicate the occurrence of Proterozoic crust east of the RBT and constrains the location of the Proterozoic–Palaeozoic boundary in Cambrian Gondwana.

U–Pb zircon geochronology and Nd isotope geochemistry of Proterozoic granitoids in the western Churchill Province: intrusive age pattern and Archean source domains

Hudsonian tectonism in the western Churchill Province of Canada encompassed two periods of granitic plutonism. The Hudson suite plutons represent xenocryst-rich, mainly monzogranitic, near-minimum melts emplaced near their source region in thickened middle crust; the Nueltin syenogranites and Pitz rhyolites were generated by intrusion of basalt into extending crust. Zircons from four Nueltin suite samples and 20 Hudson suite samples were analysed by U–Pb SHRIMP techniques. Intrusion of the Hudson suite lasted ca. 50 million years, from 1845 to 1795 Ma, with a peak around 1825 Ma, and the Nueltin suite from 1765 to 1750 Ma, following a ca. 30-million-year period of magmatic quiescence. A marked contrast in Th/U ratios was found in igneous zircon from Hudson suite samples south (Th/U = 0.066) and north (Th/U = 1.07) of the northern margin of the Central Hearne domain and is attributed to a difference in the nature of the melting process, with the lower values representing ultrametamorphism and the higher values representing bulk melting resulting from contact with mafic igneous magma. Whole-rock Nd isotopic measurements, with average ε143Nd near –10, indicate a dominant late Archean crustal component in both suites of granites, reflected by numerous inherited zircon ages between 2720 and 2650 Ma in Hudson granites. Further south, along the Manitoba border, inherited zircons of 3500–3000 Ma, together with previously published U–Pb data on gneisses and ca. 3 Ga TDM model ages of Hudson granites, indicate the presence of middle to early Archean crust.

A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe U-Pb Study

Sensitive high-resolution ion microprobe U–Pb dating shows that a biotite orthogneiss from the Hercynian belt of western central Iberia contains 1000–300 Ma zircon. Older, 1000–570 Ma ages within this range represent inherited, detrital material among which four age components may be recognized: 980 Ma, 830 Ma, 616 10(2s) Ma and 582 5(2s) Ma. This inherited zircon commonly forms cores that are surrounded by rims yielding Late Pan-African ages, identical to those found in slender, prismatic, and some stubbier, bi-pyramidal, euhedral crystals. This is the predominant type of zircon with an average age of 546 3(2s) Ma, thought to have been formed during the main magmatic crystallization stage of the granitic protolith of the gneiss. Local deuteric replacements of magmatic zircon yield a virtually identical average age of 547 5(2s) Ma, suggesting rapid magmatic cooling, typical of shallow intrusive settings. Many zircon crystals have very thin, low-Th/U rims with an age of 315 Ma, suggested to represent the gneissification of the granitic rock during the Hercynian orogeny. The abundance in the gneiss body of Al-rich restitic material and inherited, detrital zircon suggests that the granitic magma was formed by anatectic melting of a metasedimentary source rock complex. The age of the youngest inherited, detrital zircon constrains the sedimentation age of the ( youngest parts of the) anatectic source rock complex to the Late Neoproterozoic (<582 5 Ma) and leaves a maximum period of 40 Myr for metamorphism and anatexis of the source rock complex, and migration and intrusion of the granitic magma. Among the inherited, detrital zircon the 616 10(2s) Ma and 582 5(2s) Ma ages are by far the most abundant, and might be derived from the West African craton, where such ages are common for main-stage Pan-African complexes. The older, 980 Ma and 830 Ma inherited zircon ages are absent or rare in the West African craton and a derivation from the Amazon craton (Grenvillian, Sunsas orogens), where such ages are common, is an alternative.

Nd-isotopic composition of Phanerozoic sediments in the Inner Zone of Southwest Japan Arc : implications on provenance characteristics and contribution to formation of mature island arc system

The Nd-isotopic data on sedimentary and metamorphic rocks of SW Japan Arc allow their discrimination into five different depleted mantle (TDM) model age clusters, 2.6-2.45 Ga, 2.3-2.05 Ga, 1.9-1.55 Ga, 1.45-1.25 Ga, 1.2-0.85 Ga. The 2.6-2.45 Ga and 1.9-1.55 Ga model ages are also coincident with U-Pb inherited zircon ages of the above two epochs as well as the major magmatic activity in the Sino-Korean Craton (SKC). The 2.3-2.05 Ga model ages can be considered as the initial formation ages for the precursors of sedimentary rocks. The Nd-isotopic data suggest that the Hida Belt was most likely formed as a part of the SKC. The mantle underlying the Ryoke Belt had continental lithospheric signature during Triassic-Jurassic period. The 1.9-1.55 Ga model ages, especially 1.8 Ga~, can be associated with the formation of this belt. The source material for the sedimentary rocks occurring in the accretionary terrane of northeastern areas in the SW Japan Arc was probably in and around the SKC of the Ryoke Belt itself. The sedimentary rocks occurring in southwestern areas of the Arc were mainly composed of materials derived from a relatively younger source (1.45-0.85 Ga). (Communicated by Syun-iti AKIMOTO, M.J.A., Jan. 13, 2004)

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and Nd–Sr isotopic compositions and U–Pb zircon ages of two Variscan granites (Neunburg and Oberviechtach) from southern Oberpfalz, NW Bohemian massif, have been investigated in order to place constraints on their formation and on the crustal reworking. Both granites exhibit similar mineralogical, chemical and isotopic characteristics. They have peraluminous compositions (A/CNK ratios 1.2–1.3) and display high K2O/Na2O ratios of 2.2–2.3, consistent with typical S-type granites. In terms of trace elements, they show an enrichment of LREE and strong fractionation between LREE and HREE (LaN/YbN ratios 46 to 60). Compared with the primordial mantle, distinct negative anomalies of several trace elements (Ba, Sr, Nb and Ti) are also observed in both granites. They are further characterised by low initial eNd-values of −6.2 to −8.2 and high initial 87Sr/86Sr ratios of 0.7114 to 0.7147. Zircon U–Pb data indicate that the intrusion of both granites shortly post-dates the HT–LP metamorphism of the Moldanubian basement and crystallised at about 320 Ma. The samples studied contain zircons mostly having xenocrystic cores with diverse morphologies. These inherited zircons have Early Proterozoic to Early Palaeozoic ages. This points to melting of sources comprising substantial sedimentary rocks. The LaN/YbN and TbN/YbN ratios of both granites are the highest so far reported from granitoids within this region. Melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the REE characteristics.

Development of the Archaean Mallina Basin, Pilbara Craton, northwestern Australia; a study of detrital and inherited zircon ages

SHRIMP U–Pb zircon dates are combined with an examination of the age distribution patterns and provenance of both detrital zircons and of zircon xenocrysts in granites to investigate the development of the Archaean Mallina Basin, in the granite–greenstone terrain of the Pilbara Craton, northwestern Australia. The oldest dated components of the basin are c. 3010 Ma volcaniclastic rocks in the western part of the area. New data indicate that siliciclastic turbidites that dominate the southern and eastern part of the basin were deposited at or after c. 2970 Ma but before c. 2955 Ma. Linking both the detrital zircon populations as well as zircon xenocrysts from granites that intruded the Mallina Basin to well-dated areas of the Pilbara granite–greenstone terrane indicates that the sediment was derived from the south, north, northwest, and east. The basin probably evolved primarily in an intracontinental setting between two elevated land masses to the southeast and northwest. Most of the rocks within the basin were folded before intrusion of granites, the oldest of which has been dated at 2954±4 Ma . Evidence of a second depositional cycle is provided by a maximum depositional age of 2941±9 Ma , indicated by a detrital zircon population from a sample of wacke from the southeast part of the Mallina Basin. This second depositional phase may have been related to renewed extension, and recycling of sedimentary rocks within the basin.

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.

Northern Andean crustal evolution: New U-Pb geochronological constraints from Ecuador

U-Pb data obtained from granitoids and metamorphosed gabbros from the Cordillera Real and the El Oro metamorphic complex in Ecuador elucidate the pre-Cretaceous magmatic history and subsequent metamorphic evolution of the northern Andean basement. Two major tectonothermal episodes and later thermal overprinting have affected the region, obscuring the timing of plutonic and metamorphic events as constrained by K-Ar and Rb-Sr geochronology. U-Pb zircon and monazite ages show that S-type granitoids in the Cordillera Real and El Oro complex, and mafic intrusions in El Oro, were emplaced during the Triassic. The Piedras mafic intrusions, important components of the El Oro complex that were previously considered to be Precambrian, were intruded at 221 +18 / −16 Ma. The S-type Marcabeli pluton and the Limon Playa intrusion yield ages of 227.5 ± 0.8 Ma and 200 ± 30 Ma, and in the Cordillera Real, the large S-type Tres Lagunas batholith was intruded at 227.3 ± 2.2 Ma. The Marcabeli and Tres Lagunas data show that S-type granite emplacement was contemporaneous in the two belts, supporting field and geochemical evidence that the Cordillera Real and El Oro complex were contiguous in the Triassic and then separated. The granitoids constrain the age of regional shearing, metamorphism, and anatexis (Moromoro tectonothermal event) related to Gondwana–North America separation. After the Moromoro event, subduction-related calc-alkaline granite batholiths intruded the Cordillera Real. Cessation of this magmatic episode is probably constrained by the 143 ± 1 Ma Azafran batholith. Metamorphic U-Pb ages are only recorded by 78–82 Ma monazite ages from the Limon Playa granite, which are similar to many reset K-Ar ages from elsewhere in both the Cordillera Real and El Oro complex. Recycling of material originally derived from the Amazonian craton, together with younger Cambrian–Neoproterozoic crust, is suggested by single grain inherited zircon ages of 546 Ma, 2220 Ma, and 2876 Ma from the Marcabeli pluton, together with Neoproterozoic and Mesoproterozoic upper intercept ages for Tres Lagunas and Limon Playa zircons. The zircon data are supported by 1400–1600 Ma Nd-depleted mantle model ages (T DM ) for both the Tres Lagunas granite and Paleozoic migmatitic gneisses from El Oro and Cordillera Real. Plate tectonic reconstructions supported by paleontological and isotope data indicate that paleo-Mexico and the Colombian segment of the Andes were joined prior to the breakup of Pangea. Additional evidence supporting this scenario is given by the overlap of T DM ages for granitoids and metasedimentary rocks in the Acatlan complex of Mexico and the Cordillera Real and El Oro complex; these rocks serve as a suitable proxy for related lithologies in Colombia, the Nd T DM ages of which are unknown.

Additional Rb-Sr and single-grain zircon datings of Caledonian granitoid rocks from Albert I Land, northwest Spitsbergen

Previous K-Ar and Rb-Sr datings of the metamorphic and granitic rocks from the northwestern basement region of Spitsbergen mainly show the cooling time of the rocks, except for a Rb-Sr isochron age of the Hornemantoppen granitoid. New samples were collected during several years of geological mapping in the area and the Rb-Sr whole rock isochron and single-grain zircon evaporation methods were applied to the Hornemantoppen granitoids and the grey granites. A dioritic dyke was also dated by the latter method. The bulk rock chemistry study shows that most of both granitic rocks are of the S-type and probably post orogenic, with distinctive incorporation of crustal materials. The isotopic data also support this interpretation. The results of the Rb-Sr isotope analyses, 412 ± 4.8 Ma and the zircon Pb evaporation age of 424 ± 56 Ma, confirm the previous age of the Hornemantoppen granitoid, 414 ± 10 Ma. An older zircon age of 547 ± 19 Ma is considered to be the minimum age of inherited zircon. Zircons from the grey granites suggest an age of ca. 420 Ma with a large error. Field relations demonstrate that the grey granites are older than the Hornemantoppen granitoids. A minimum inherited zircon age, 952 ± 20 Ma, has been obtained from the grey granites. Three multi-grain Pb ages, 423 ± 22 Ma (2 grains), 461 ± 42 Ma and 561 ± 93 Ma (the last two 3 grains) were considered to be mixed ages. Although no definitive evidence for the presence of Grenvillian granites in this area has been obtained in the present study, preliminary results from the multi-grain zircon evaporation method, carried out in the Russian laboratory at Apatity, infer Paleo- and Mesoproterozoic protoliths for the metamorphic rocks of northwestern Spitsbergen.