Showing papers on "Metamorphism published in 2013"

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

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

A ∼2.5 Ga magmatic event at the northern margin of the Yangtze craton: Evidence from U-Pb dating and Hf isotope analysis of zircons from the Douling Complex in the South Qinling orogen

Abstract: The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt. Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating, which yielded protolith emplacement ages of 2469 ± 22 Ma, 2479 ± 12 Ma, 2497 ± 21 Ma, 2501 ± 17 Ma and 2509 ± 14 Ma, respectively. An important peak age of ∼2.48 Ga was also obtained for a metasedimentary rock in the same region. These discoveries suggest the occurrence of magmatic activity of 2.51–2.47 Ga at the northern margin of the Yangtze craton. The age-corrected ɛHf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between −5.5 and +0.3, and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga. Considering two important periods of ∼3.3–3.2 Ga and ∼2.95–2.90 Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo- to Mesoarchean crust at the northern margin of the Yangtze craton at ∼2.5 Ga. In addition, metamorphic ages of 837 ± 8 Ma and 818 ± 10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock, indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic, although its geological meaning remains ambiguous.

Early Cretaceous sedimentation and orogeny on the active margin of Eurasia: Southern Central Pontides, Turkey

Abstract: [1] The Pontides in northern Turkey constituted part of the southern active margin of Eurasia during the Mesozoic. In the Early Cretaceous, a large submarine turbidite fan covered most of the Central Pontides. New U-Pb detrital zircon data imply that the major source of the turbidites was the East European Craton-Scythian Platform in the north. This implies that there was no thoroughgoing Black Sea basin between the Pontides and the East European Craton during the Early Cretaceous. The Lower Cretaceous turbidites are bounded in the south by a large metamorphic area, the Central Pontide Supercomplex (CPS). New geological mapping, petrology, and U-Pb zircon and Ar-Ar muscovite ages indicate that the northern part of the CPS consists of Lower Cretaceous distal turbidites deformed and metamorphosed in a subduction zone in the Albian. The rest of the CPS is made of Middle Jurassic, Lower Cretaceous, and middle Cretaceous (Albian) metamorphic belts, each constituting distinct subduction-accretion units. They represent episodes of collision of oceanic volcanic arcs and oceanic plateaus with the Eurasian margin and are marked in the stratigraphy of the hinterland by periods of uplift and erosion. The accretionary complexes are overlain by Upper Cretaceous (Turonian-Santonian) volcano-sedimentary sequences deposited in a fore-arc setting. The detrital zircon data, middle Cretaceous (Albian) metamorphism, and widespread Albian uplift of the Black Sea region suggest that Early Cretaceous (Barremian-Aptian) nonvolcanic rifting and Late Cretaceous (Turonian-Santonian) opening of the Black Sea by the splitting of the arc are unrelated events.

Subduction along and within the Baltoscandian margin during closing of the Iapetus Ocean and Baltica-Laurentia collision

Abstract: The recent discovery of ultrahigh-pressure (UHP) mineral parageneses in the far-transported (greater than 400 km) Seve Nappe Complex of the Swedish Caledonides sheds new light on the subduction system that dominated the contracting Baltoscandian margin of continental Baltica during the Ordovician and culminated in collision with Laurentia in the Silurian to Early Devonian. High-grade metamorphism of this Neoproterozoic to Cambrian rifted, extended, dike-intruded outer-margin assemblage started in the Early Ordovician and may have continued, perhaps episodically, until collision of the continents at the end of this period. The recent discovery of UHP kyanite eclogite in northern Jamtland (west-central Sweden) yields evidence of metamorphism at depths of 100 km. Although UHP rocks are only locally preserved from retrogression during the long-distance transport onto the Baltoscandian platform, these high-pressure parageneses indicate that deep subduction played an important role in the tectonothermal history of the complex. Based on existing isotopic age data, this UHP metamorphism occurred in the Late Ordovician, shortly before, or during, the initial collision between the continents (Scandian orogeny). In some central parts of the complex, migmatization and hot extrusion occurred in the Early Silurian, giving way to thrust emplacement across the Baltoscandian foreland basin and platform that continued into the Early Devonian. Identification of HP/UHP metamorphism at different levels within the Scandian allochthons, definition of their pressure-temperature-time paths, and recognition of their vast transport distances are essential for an understanding of the deeper structural levels of the orogen in the hinterland (e.g., the Western Gneiss Region), where the attenuated units were reworked together during the Early Devonian.

Petrochronology of Himalayan ultrahigh-pressure eclogite

Abstract: The timing and nature of the India-Asia collision, Earth’s largest ongoing continent-continent collisional orogen, are unclear. Ultrahigh-pressure metamorphism of Indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of India. Records of this metamorphism are preserved only at Kaghan Valley (Pakistan) and Tso Morari (Ladakh, India), separated by ~500 km and having published ages of peak pressure of 46.2 ± 0.7 Ma and 53–51 Ma, respectively. The apparent ~6 m.y. age difference may refl ect multiple subduction events, a large promontory along the former Indian margin, or inadequate constraints on the time of peak pressure recrystallization at Tso Morari. We present 108 coupled, in situ U/Th-Pb and rare earth element (REE) analyses of zircons in two Tso Morari eclogites to obtain age and petrologic information. The ages range from ca. 53 Ma to 37 Ma, and peak at ca. 47–43 Ma. Flat heavy REE slopes and the absence of an Eu anomaly are compatible with eclogite-facies zircon (re)crystallization. This (re)crystallization probably occurred at ultrahigh pressure, because 64% of the analyses are from zircon included in ultrahigh-pressure garnet and omphacite. These results are consistent with those from Kaghan Valley, and suggest that a single, protracted ultrahigh-pressure metamorphic event occurred contemporaneously across much of the orogen, following initial contact of the Indian and Asian continents at ca. 51 Ma or later.