Thursday, January 5, 2017

Photomicrograph: Authigenic Feldspars From The Neoproterozoic - South India

Feldspars (plagioclase and alkali feldspars) are the most common minerals in the earth's crust. The vast bulk of them crystallize out of magma and lava. Feldspar also forms during metamorphic reactions. In sedimentary rocks they are commonly seen as detrital grains in sandstones. What is less appreciated is that they can also grow de novo in sediments during diagenesis i.e. during chemical reactions that take place as loose sediment reacts with fluids and gets transformed into rock.

  Authigenic twinned euhedral feldspar cross cutting mud clast

I noticed some lovely examples of such diagenetic or authigenic feldspars from the Neoproterozoic Banganapalli Formation from the Cuddapah Basin in South India during my M.Sc. dissertation project work. I recently got a chance to photograph my old thin sections again and I am posting some more photomicrographs of these authigenic feldspars below.
The Banganapalli Formation also termed the Banganapalli Quartzite is made up mostly of conglomerates and sandstones. They rest with an unconformable contact on the Paleoproterozoic Tadpatri Shale. There is spatial variability in the composition of the Banganapalli sediments. In my study area south of the village of Gani in Andhra Pradesh, the conglomerates and sandstone interfinger with limestones. These limestones appear a light purple in outcrop and are made up of carbonate mud with intermittent conglomerate layers and lenses of quartzite and jasper pebbles and cobbles, thin bands and layers of quartz sand along with limestone and siliciclastic mud intraclasts often showing a chaotic fabric (left). Fine clay layers are dispersed through the succession.

Near the contact between the Banganapalli limestones and the overlying Narji Limestone is an  intraclast conglomerate layer (right) made up of carbonate and siliciclastic intraclasts indicating rapid lithification of the sea floor and the subsequent disruption of hardened sea floor crusts during storms and seismic events.

The authigenic feldspars are present in this Banganapalli limestone succession. Feldspars are euhedral (well formed facets) and show contact twinning (separate crystals grow symmetrically forming mirror images across a common plane). They contain inclusions of calcite and clay. Mineral composition studies have shown that authigenic feldspars are either albite (sodium alumimium silicate) or orthoclase (potassium aluminium silicate). At that time (in the late 1980's)  I did not have access to an electron microprobe to accurately ascertain the composition of these feldspars. The twinning exhibited by these feldspars suggests to me that these are albite.

The feldspars cross cut intraclasts (arrow)

and calcite veins (arrow)

They grow in the carbonate mud matrix (arrow)

When did they form?

Feldspar crystals cross cut fractures and veins filled with calcite. This implies that they formed after lithification of the sediment. This could have occurred very early in the sediment history. Sea floor cementation and lithification of carbonate is commonly observed from the Proterozoic, which had calcium carbonate supersaturated oceans. The presence of intraclast layers with chaotically oriented clasts and cracks filled with detrital silica pebbles and sand indicate early lithification and disruption of sea floor. Calcite veins may simply suggest hardening and breakage of lithified sediment layers.  Sea water percolating through cracks in this early lithified sediment would have supplied sodium to the growing feldspars.

Alternatively,  the feldspars formed later under burial conditions. The Banganapalli Formation is made up of immature sandstone bodies containing plagioclase and alkali feldspar detrital grains. They show signs of dissolution and corrosion during diagenesis (in image below arrows point to partially dissolved feldspar grains).

Alkali released from the dissolution of detrital feldspars was transported by groundwater flow and used up in the growth of authigenic feldspars in adjacent limestones.

I'll leave the question open.

There are other interesting diagenetic features too in these sediments. The siliciclastic mud intraclasts show alteration to chlorite and glauconite and there is extensive neomorphic recrystallization of carbonate mud.

Authigenic feldspar are reported from sandstones too. They usually occur as tiny overgrowths on detrital feldspar grains. They are less well known from limestones. So, these unusually large euhedral authigenic feldspars stole the show for me.

Finally, the satellite image below shows the location of the limestone layers (black arrow) containing these authigenic feldspars. They occur on the south dipping limb of the Gani-Kalava anticline near the town of Nandayal.

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