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

Abstract

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.

Pressure‐temperature‐time evolution of ultrahigh‐temperature granulites from the Trivandrum Block, southern India: Implications for long‐lived high‐grade metamorphism

“…1,130 to 930 Ma, during a single metamorphic event. Similar long‐lived high‐grade metamorphism has also been reported from Arunta region (Morrissey et al, ) and Musgrave Province (Walsh et al, ) in Central Australia, and Southern Madagascar (Fitzsimons, ). However, detailed studies on the duration of metamorphism in the Trivandrum Block are still limited (e.g., Taylor et al, ).…”

“…They regarded that such long‐lived conductive heating was provided by high crustal heat production in voluminous premetamorphic granitic rocks. Fitzsimons () suggest that metamorphism in southern Madagascar was driven by radiogenic heat accumulation beneath a long‐lived orogenic plateau because of the 50 Myr duration of extreme P–T conditions and lack of coeval mantle magmatism.…”

“…Several studies have attempted to construct the pressure‐temperature‐time ( P–T–t ) paths of high‐grade metamorphism from various granulite‐facies terranes in the world for quantitative arguments of metamorphic processes and regional tectonic correlations (e.g., Clark, Collins, Santosh, Taylor, & Wade, ; He et al, ; Takamura, Tsunogae, Santosh, Malaviarachchi, & Tsutsumi, ). Particularly, the duration of granulite‐facies metamorphism including those at UHT conditions has been widely discussed in many recent studies (e.g., Fitzsimons, ; Harley, ; He et al, ; Kelsey & Hand, ; Korhonen, Clark, Brown, Bhattacharya, & Taylor, ; Morrissey, Hand, Raimondo, & Kelsey, ; Walsh et al, ) because the timescale is critical to models on tectonic settings and relationships to supercontinent assembly (Harley, ). Detailed studies from several well‐documented granulite‐UHT terranes reveal long duration metamorphism, with UHT (>900°C) rocks suggesting a range of 30–100 Myr.…”

Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar

“…Many studies point toward the latter, as this mechanism can accommodate collapse without surface extension being necessary (Rey et al, , and references therein). The Tibetan plateau is the best modern analogue for the Pan‐African collision zone (Fitzsimons, ; Horton et al, ). Radial anisotropy of the Tibet/ Himalaya region was examined in several studies (e.g., Chen et al, ; Duret et al, ; Guo et al, ; Huang, ; Shapiro et al, ).…”

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

“…Identification of regional UHT metamorphism that persisted for >100 Ma is becoming more common, with examples including the Eastern Ghats Province (Kelsey et al., ; Korhonen et al., ), the East African Orogen (Clark et al., ; Fitzsimons, ; Harley, ; Horton et al., ) and the Musgrave Orogen (Walsh et al., ). There are relatively few large‐scale, long‐lived heat sources available to sustain UHT in the crust for >100 Ma.…”