Thursday, June 1, 2017

The Serpents Of Nagling- Granite Intrusions Into Greater Himalayan Sequence Metamorphics

Over chai, elders told us about large serpents invading their village. A curse, they said. Only the correct prayers and purification rituals saved them, forcing the serpents to retreat deep into the forest. Some serpents remain trapped in the rock faces near the village, which was renamed Nagling (Nag means cobra..or more generically serpent).

The picture below are the entombed serpents of Nagling (trekkers for scale).


Geologists recognize them to be granite dykes (intrusions cutting across host rock layering) and sills (intrusions parallel to host rock layering) intruding the high grade metamorphic rocks of the Greater Himalayan Sequence (GHS).

The GHS is a block of the Indian crust bounded between the Main Central Thrust (MCT) at the base and the South Tibetan Detachment System (STDS) at the top. It represents mid crustal material which was metamorphosed and then was extruded and exhumed during Himalayan orogeny between 25 million years ago to about 16 million years ago. These dates vary somewhat along the strike of the Himalaya. Thrusting along the MCT took place earlier in the western Himalaya. Eastern regions like the Sikkim Himalaya record younger dates for the movement of the MCT.

The grade of metamorphic varies within the GHS. The figure below is a schematic section of the Greater Himalayan Sequence. It is from a study on the nature of the MCT by Michael Searle and colleagues from the Nepal Himalaya and is a very useful guide to think about the internal structure of the GHS.


 Source: Searle et. al. 2008

From the base of the MCT the grade of metamorphism increases towards higher structural levels. This is recognized as an "inverted metamorphic gradient", since minerals that are formed at higher and higher temperatures and pressures are occurring at structurally higher and by implication apparently shallower levels of the crust. The inverted gradient is recognized by the successive appearance of  biotite, garnet, sillimanite and finally kyanite. The sillimanite-kyanite zone transitions into the zone of partial melting and granite intrusives. This is the zone where the crust experienced conditions that lead to the formation of in situ melts and their mobilization and intrusion into surrounding rock. Above this zone the grade of metamorphism reduces towards the STDS. In the figure, the granite intrusion zone is directly overlain by the STDS and the Tethyan sequence. However, there is variation in this theme across the Himalaya. In the Kumaon region where I was, the "melt zone" is overlain by a sequence of lower metamorphic grade phyllite rocks.

What caused this melting and production of granitic magma? Many geologist point to the STDS. They suggest that this zone of extentional faulting stretched and thinned the crust, resulting in " decompression-related anatexis". This means that when extentional faulting along the STDS and exhumation reduced the overburden on deeply buried hot rocks, the release in pressure resulted in the lowering of rock melting point. This led to a partial melting of the crust (anatexis). Other geologists disagree with this explanation. They point out that since decompression has a minor effect on melting the likely source rock compositions you would require unreasonably large amounts of denudation along the STDS.  Rather, they suggest that crustal thickening by the continued convergence of India with Asia elevated temperatures in the middle levels of the crust to a range where partial melting began. These melts then moved along weak planes and intruded the surrounding GHS above the sillimanite and kyanite grade gneisses. The main pulses of this magma generation took place between 24 million years and 19 million years ago.

Geologists estimate the temperatures of this melt zone to be around 650 deg C to 750 deg C, corresponding to a  burial depth of about 20-25 km. Yes, the GHS represents crust that has traveled from that depth to the Himalayan heights it now commands by a combination of thrust faulting and erosional unroofing i.e. the stripping away of shallower levels of the crust!

During one of my previous treks in the Kumaon region I had walked across the GHS from the base of the MCT to the sillimanite zone in the Goriganga valley from the town of Munsiari to village Paton. This time, one valley to the east,  we began our trek at village Nagling in the zone of  partially melting. All around us were rock faces intruded by sill complexes and dykes. The picture below shows multiple sills of granite cross cut by dykes.


High up from Nagling village towards Nagling Glacier I saw this granite dyke complex (outlined by red dotted lines ) cutting across metamorphic banding (black lines).


And in the stream near Nagling Glacier I came across this rounded stream boulder showing granite cross-cutting banded migmatitic gneiss.


We traveled north and  reached Duktu. Earlier, somewhere near the village of Baaling, we had crossed the zone of partial melting and were in the uppermost levels of the GHS made up of phyllite grade metamorphic rocks. The phyllites are not intruded by granite.

However, granite was present at Duktu too, but only in the Dhauliganga river bed. This river emerges from the Panchachuli Glacier. The Panchachuli ranges which fall lower in the GHS are made up of high grade gneiss intruded by granite.

As a result, the Dhauliganga river bed near Duktu village is choked with boulders of granite and migmatite rocks.


This is a very distinctive  biotite-tourmaline granite. The picture below shows blocks of granite with tabular black tourmaline.


Here is a picture of me looking intently at a block of GHS made up of a granite intruding in to a gneiss.


And another close up of light colored granite intruding dark grey banded gneiss and encircling and enclosing rafts of the metamorphic host rock (red arrows).


And finally, from the sheer rock faces near Nagling Glacier, one of my favorite examples of the granite intrusions. A near vertical dyke (red broken outline) cut and displaced by a fault (yellow broken lines). Metamorphic banding shown in black lines.


... Pleistocene-Holocene glacial deposits of the Panchachuli Glacier area.. coming up next!

4 comments:

  1. Another neat post from that trip ... I enjoy it so much when legend and science converge!

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  2. yes, It was fun to listen to stories from the village elders, sipping hot chai in a smoky wood fired kitchen! great atmosphere to then relate that to geology.. :)

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  3. Hi Suvrat! Just read this again, and understood more... Thanks very much for this. I look forward to your next one on the Pleistocen-Holocene deposits too. For now, just a small input from me, on the 'cultural' aspect. Nagling, which would translate literally to mean the phallus of a snake, is actually a recent story, and one probably to match either a lazy tongue, or more probably, cultural camouflage. The name of the village (and the mountain at the head of the valley was actually something closer to Ngangling, which is probably a tibetan word. In both the Darma and the Gori valley, people of Tibetan descent living there find it politically and culturally expedient to deny their Tibetan ancestry and especially their Buddhist past. There are many instance and details I can cite, and that you may have encountered when you traveled here. Just one other example is the pass between the Ralam and the Gori valleys up north,probably a tibetan name Burjikang, but now called Brijganga! Did you see the rock with Tibetan script in Baling? There are still a few here in Martoli, Burfu and Milam...
    Thanks a mill again...

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    1. Theo-

      thanks for that very interesting insight! we came across lots of people of Tibetan descent, including the guy from whom we rented horses. There seems to be a fair amount of intermarriage between Tibetan and the locals. Besides other political currents, that is one sure way towards the loss (usually of the minority) of cultural and linguistic identity. Yes, we did see the rock with Tibetan script..

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