/pdf/insights-into-the-tectonic-evolution-of-the-north-china-4npdhca6nf.pdf

Insights into the tectonic evolution of the North China Craton through comparative tectonic analysis: A record of outward growth of Precambrian continents

The available plate-tectonic evolution models suggest that the deep ocean basins adjoining the western continental margin of India have evolved largely due to break-up and dispersal of India, Seychelles and Madagascar continental blocks since Late Cretaceous. Mainly owing to the availability of large number of well identified magnetic anomaly picks, the evolution of the region from chron C28ny (~62.5 Ma) and younger times is better constrained than the preceding period of its early evolution. Using constraints of several recently mapped regional scale tectonic features, a plausible model for that early evolution is proposed in this paper. Around 88.0 Ma the involved continental blocks were in their immediate pre-drift configuration where a wide continental rift zone existed between India and Madagascar. Seafloor spreading in the Mascarene Basin commenced shortly before 83.0 Ma. A ternary rift system off Saurashtra peninsula of western India, formed shortly before 68.5 Ma, reached seafloor spreading stage in the Laxmi and Gop basins around 67.6 and 64.7 Ma respectively. Around 62.5 Ma the ancestor of the Carlsberg Ridge spreading center developed between the Seychelles Plateau and the Laxmi Ridge while spreading in the northern Mascarene Basin ceased and spreading in the Laxmi and Gop basins continued at very slow rate. Between 60.9 and 57.9 Ma the spreading in the southern Mascarene Basin also ceased and the spreading center jumped north between the Laccadive Plateau and the northern boundary of the Mascarene Basin. The divergence regimes of the Gop, Laxmi and Laccadive basins ceased between 57.6 and 56.4 Ma, and the Laccadive Plateau and the Laxmi Ridge got welded to the Indian plate.

Plate-Tectonic Evolution of the Deep Ocean Basins Adjoining the Western Continental Margin of India—A Proposed Model for the Early Opening Scenario

Paleoproterozoic metamorphism of high-grade granulite facies rocks in the North China Craton: Study advances, questions and new issues

We have compared different spectral methods (the standard windowed Fourier transform, the multitaper method (MTM), and the maximum entropy method (MEM)) to determine the coherence between Bouguer gravity and topography and estimate the variations of the effective elastic thickness (Te) of the lithosphere. These methods differ significantly in spatial resolution (1000 to 2000 km) determined by the width of the windows required to resolve the wavelength of transition from low to high coherence. The tests with synthetic data show that the standard deviation on elastic thickness estimates is high with the three methods but that the mean value obtained by the maximum entropy is close to the input value, while the other methods underestimate the elastic thickness. In the Canadian Shield, the effective elastic thickness values vary from c40 km to N100 km over distances less than the width of the moving window. There is no clear geographical or geological pattern in these variations as high and low elastic thickness values are found near the edge as well as in the center of the continent, and in all provinces regardless of their age. Correlation between Te and the heat flow field is weak. The values of Te being much higher beneath Hudson Bay than the Williston Basin are consistent with models of basin evolution and seismic estimates of mantle temperatures. D 2004 Elsevier B.V. All rights reserved.

Variations in Elastic Thickness in the Canadian Shield

Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25–50 °C km−1 were attained at 0.58–0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure–temperature (P–T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long–lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India–Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15–25 °C km−1 but with quite different ages. Metamorphism along the western suture at 0.65–0.61 Ga followed the end of magmatism in an adjacent 0.85–0.65 Ga ocean–arc terrane. It has an anti–clockwise P–T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53–0.51 Ga has a clockwise P–T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

India–Madagascar paleo-fit based on flexural isostasy of their rifted margins

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The Ranomena ultramafic complex in NE Madagascar consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered peridotite. This study of the Ranomena chromite chemistry aims to better understand the petrogenesis and palaeotectonic environment of the complex. The chromite from the Ranomena chromitite is unzoned/weakly zoned and has a Cr# (Cr/(Cr + Al)) of 0.59-0.69, a Mg# (Mg/(Fe + Mg)) of 0.37-0.44, and low Al2O3 (15-23 wt %) suggesting derivation from a supra-subduction zone arc setting. Calculation of parental melt composition suggests that the parental magma composition of the Ranomena chromitite was similar to that of a primitive tholeiitic basalt formed at a high degree of mantle melting, suggesting the parental melt composition was equivalent to that of an island-arc tholeiite (IAT). The parental magma of the Ranomena chromite had a FeO/MgO ratio of 0.9 to 1.8, suggesting arc derivation. The parental magma was Al- and Fe-rich, similar to a tholeiitic basaltic magma. The composition of orthopyroxene from the chromitite indicates a crystallization temperature range of 1250-1300 degrees C at 1.0 GPa. The chemistry of the chromite in the Ranomena chromitite further suggests that the complex formed in a supra-subduction zone arc tectonic setting.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/8F12ACE7229C962D7E7255BD9C5D516E/S0016756816000972a.pdf/div-class-title-chromite-chemistry-as-an-indicator-of-petrogenesis-and-tectonic-setting-of-the-ranomena-ultramafic-complex-in-north-eastern-madagascar-div.pdf

T. Razakamanana

Papers

India–Madagascar paleo-fit based on flexural isostasy of their rifted margins

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

Chromite chemistry as an indicator of petrogenesis and tectonic setting of the Ranomena ultramafic complex in north-eastern Madagascar

Petrogenesis and tectonic setting of the Bondla mafic-ultramafic complex, western India: Inferences from chromian spinel chemistry