M. V. Luchitskaya

Age of granitoid plutons, North Chukotka: Problem formulation and new SHRIMP U-Pb zircon datings

A brief review of presentday ideas about the age ofgranitoid plutons in the Chauna zone of the Verkhoyansk–Chukotka fold belt and results of SHRIMP U–Pb zircon dating in two large plutons, Velitkenai andMoltykan, are presented. In spite of structural differences, both plutons are characterized by a similar iso

Campanian stage of granite formation in the south of the Sredinnyi Range in Kamchatka: New U-Pb SHRIMP Data

This article gives an account of the results of the U-Pb-SHRIMP study of zircons derived from gneissoid and equigranular granitoids of the Malka Uplift of the Sredynnyi Range in Kamchatka. It was established that intrusion and crystallization of granitoids occurred in the time interval from 76.2 ± 1.5 to 83.1 ± 2.0 Ma. The texture of zircon crystals suggests their magmatic origin. The obtained data reliably confirm that granite formation and emplacement of the recently formed continental crust in Kamchatka took place in the Late Cretaceous (Campanian).

New SHRIMP U-Pb Age Data on Zircons from Plagiogranites in the Ophiolites of the Kamchatsky Mys Peninsula, Eastern Kamchatka

Geological setting. Kamchatsky Mys peninsula has composite fold-and-thrust structure, constructed by Cretaceous and Paleocene-Eocene volcanic and terrigenous-tuffaceous rocks and tectonic slices of serpentinite mélange, gabbroids and ultrabasic rocks [Khotin, 1976; Zinkevich et al., 1985; Rasnitsyn et al., 1985; Fedorchuk, 1989; Shapiro, 1987; Accretional ... , 1993; Boyarinova et al., 2000, 2001; Saveliev, 2004]. Its southern part (Afrika block) according to [Zinkevich et al., 1985] is constructed by four allochtonous complexes. They are composed of: 1) Aptian-Albian and Albian-Cenomanian calcareouscherty-effusive assemblages (Afrika complex), 2) Campanian-Maastichtian cherty-tuffaceous and terrigenous deposits, 3) Paleocene-Lower Eocene chert-volcanic sequence (Kamensk complex), 4) serpentinite mélange and gabbroids. Serpentinite mélange contains unaltered ultrabasic rocks (Mountain Soldatskaya massif) and smaller bodies of gabbroids, Cretaceous and Paleogene tuffaceous and cherty rocks. Ophiolite fragments at Kamchatsky Mys peninsula are represented by gabbroids of Olenegorsk pluton and ultrabasic rocks of Soldatsky massif, by blocks of gabbroids with plagiogranites in serpentinite mélange, different basalts and calcareous-jasper-cherty deposits of Aptian-Cenomanian Afrika complex and tholeiite basalts, mudstones of Paleocene-Eocene Kamensk complex.

Neoproterozoic granitoids on Wrangel Island

Based on geochronological U–Pb studies, the age of Wrangel Island granitoids was estimated as Neoproterozoic (Cryogenian). Some granitoids contain zircons with inherited cores with an estimated age of 1010, 1170, 1200, and >2600 Ma, assuming the presence of ancient (Neoarchean–Mesoproterozoic) rocks in the Wrangel Island foundation and their involvement in partial melting under granitoid magma formation.

Carboniferous Granitoid Magmatism of Northern Taimyr: Results of Isotopic-Geochemical Study and Geodynamic Interpretation

Data on the petrography, geochemistry, and isotopic geochronology of granites from the northern part of the Taimyr Peninsula are considered. The Early–Middle Carboniferous age of these rocks has been established (U–Pb, SIMS). Judging by the results of 40Ar/39Ar dating, the rocks underwent metamorphism in the Middle Permian. In geochemical and isotopic composition, the granitic rocks have much in common with evolved I-type granites. This makes it possible to specify a suprasubduction marginal continental formation setting. The existence of an active Carboniferous margin along the southern edge of the Kara Block (in presentday coordinates) corroborates the close relationship of the studied region with the continent of Baltia.

Marginal continental and within-plate neoproterozoic granites and rhyolites of Wrangel Island, Arctic region

The paper presents new data on the U–Pb zircon age, as well as results of isotopic geochemical analysis, of granites and rhyolites from Wrangel Island. The U–Pb age estimates of granites and rhyolites are grouped into two clusters (~690–730 and 590–610 Ma), which imply that these rocks crystallized in the Late Neoproterozoic. Granitic rocks dated back to 690–730 Ma are characterized by negative εNd(t) values and Paleoproterozoic Sm–Nd model age. The older inherited zircons corroborate the ancient age of their crustal source. The granitic rocks pertain to involved peraluminous granites of type I, which form at a continental margin of the Andean type and can be compared with coeval granites and orthogneisses from the Seward Peninsula in Alaska. Rhyolites and granites ~590–610 Ma in age are distinguished by a moderately positive εNd(t) and Mesoproterozoic model age. It is suggested that they have a heterogeneous magma source comprising crustal and mantle components. The geochemical features of granites and rhyolites correspond to type A granites. Together with coeval OIB-type basalts, they make up a riftogenic bimodal association of igneous rocks, which are comparable with orthogneisses (565 Ma) and gabbroic rocks (540 Ma) of Seward Peninsula in Alaska.

Composition, Age, and Origin of Cretaceous Granitic Magmatism on the Eastern Chukchi Peninsula

New geochronological and isotopic geochemical data are given, which make it possible to recognize two types of granitic rocks on the eastern Chukchi Peninsula. Early Cretaceous Tkachen and Dolina granitic plutons with zircon ages (U–Pb SIMS) of 119–122 and 131–136 Ma are related to the first type. They cut through Devonian–Lower Carboniferous basement rocks and are overlain by the Aptian–Albian Etelkuyum Formation. Basal units of the latter contain fragments of granitic rocks. Late Cretaceous Provideniya and Rumilet granitic plutons, which contain zircons with ages of 94 and 85 Ma (U–Pb SIMS), respectively, belong to the second type. They cut through volcanic–sedimentary rocks of the Etelkuyum and Leurvaam formations pertaining to the Okhotsk–Chukotka Volcanic Belt. In petrographic and geochemical features, the Early Cretaceous granitic rocks of the Tkachen Pluton are commensurable with I-type granites, while Late Cretaceous granite of the Rumilet Pluton is comparable to A2-type granite. The Sr–Nd isotopic data provide evidence that from the Early Cretaceous Tkachen and Dolina plutons to the Late Cretaceous Provideniya and Rumilet plutons, the degree of crustal assimilation of suprasubduction mantle-derived melts increases up to partial melting of heterogeneous continental crust enriched in rubidium. An unconformity and various degrees of secondary alteration of volcanic–sedimentary rocks have been established in the Okhotsk–Chukotka Volcanic Belt, and this was apparently caused by transition of the tectonic setting from suprasubduction to a transform margin with local extension.

New Nd and Pb isotope data on the ophiolite complexes of the Kronotskii paleoarc, Eastern Kamchatka

The problem of typification of tectonically disintegrated ophiolite complexes in the accretionary framing of the northwestern Pacific remains a matter of debate. In addition to detailed geological, mineralogical, and geochemical data, the isotopic characteristics of the rocks of ophiolite complexes play an important role in solving this problem. We studied ophiolites of the Eastern peninsulas of Kamchatka (Kamchatskii Mys and Kronotskii peninsulas), which are the fragments of the Late Cretaceous‐Eocene Kronotskii paleoarc (Fig. 1). In order to obtain the isotopic characteristics of the ophiolite complexes, we studied Pb and Nd isotopic ratios in the least altered rocks. This paper presents new data on the isotopic composition of ophiolites of the Kronotskii paleoarc. The 143 Nd/ 144 Nd, 208 Pb/ 204 Pb, 206 Pb/ 204 Pb, and 207 Pb/ 204 Pb isotopic ratios were analyzed on a Triton TI solid phase multichannel mass spectrometer at the Center of Isotopic Research (Karpinskii All-Russia Research Institute of Geology, St. Petersburg) using the standard technique. The average reproducibility of analyses during determination of the Nd isotopic composition was no worse than 0.005% (2 σ ) for the 143 Nd/ 144 Nd ratio. The isotopic composition of the Pb standard measured during the analytical works was as follows: 143 Nd/ 144 Nd = 0.512112 ± 0.000002 for JNdi-1 standard; 206 Pb/ 204 Pb = 16.937 ± 0.011, 207 Pb/ 204 Pb = 15.492 ± 0.017, 208 Pb/ 204 Pb = 36.722 ± 0.017 for NIST-981 standard. We united ophiolite fragments in the Kamchatskii Mys Peninsula into three ophiolite complexes developed in the Afrikan block [1]: (1) Aptian‐Cenomanian oceanic complex represented by tholeiitic basalts, jaspers, and limestones of the Afrikan Complex; (2) Late Cretaceous suprasubduction complex including ultrabasic rocks of Mt. Soldatskaya and Ol’khovsk gabbro‐ plagiogranite massif; (3) Paleocene‐Eocene intra-arc complex including basalts, jaspers, and mudstones of the Kamensk Complex and gabbroids of the Olenegorsk Complex. The Stolbovskii block of the Kamchatskii Mys Peninsula contains Maastrichtian islandarc tholeiites of the Tarkhovka Subformation (Stolbovskaya Group), which are likely complementary to suprasubduction ophiolites of Mt. Soldatskaya. Similar island-arc volcanic rocks occur as tectonic blocks in the