Intrusive Contact or Basement? Mapping a Dharwar Schist Belt Margin
A mapping party works a 2 km × 3 km window 12 km southwest of Chitradurga at 1:10,000. The west-to-east traverse crosses chlorite–actinolite schist (metabasalt) with preserved pillow selvedges and graded beds that consistently fine eastwards; then a 200 m wide, coarse pink granite whose sharp contact cuts obliquely across the schistosity at about 40°; then schist again. Within 15 m of the contact the granite carries angular amphibolite fragments up to 30 cm across, several with the same internal fabric as the country rock. No conglomerate, weathered profile or basal sandstone appears anywhere along the contact. A teammate argues that the granite must be the older basement 'because in the Dharwar craton the gneiss–granite is the basement and the schist belts rest on it', and on that basis wants the pillows read as facing west. The contact description must go into the field report tonight.
1. Is the contact intrusive or an unconformity? Rank the observations by how much weight they carry.
Read the worked answer
Intrusive. Ranked evidence:
- Angular wall-rock inclusions carrying the country-rock fabric (highest weight). An igneous body that engulfs solid, foliated amphibolite has invaded it. An unconformity produces rounded clasts in a sedimentary bed, not angular blocks floating in coarse granite.
- The contact cuts the schistosity at ~40°. An erosion surface does not slice a regional fabric obliquely for 200 m while the overlying rock has no fabric at all.
- No basal conglomerate, palaeosol or weathered horizon. A nonconformity in this terrain should show at least a rubble/regolith zone.
- The granite is undeformed and homogeneous while the schist belt is folded - so the granite post-dates the deformation.
The teammate has confused the old Peninsular Gneiss basement (about 3.0–3.4 Ga) with this younger, pink, undeformed intrusive of Closepet type (about 2.5 Ga). Write it as an intrusive contact; the granite does not belong in the belt's stratigraphic column at all.
2. Name the two field checks that would settle the question beyond reasonable doubt, and say what result each would give.
Read the worked answer
Check 1 - trace the contact and look for apophyses. Walk the contact for several hundred metres and map its trace. An intrusive contact sends dykes, sills and tongues of granite into the schist, has an irregular, lobate trace, and often shows a finer-grained chilled margin against the wall rock. An unconformity is a single, comparatively straight or gently undulating surface with nothing branching from it.
Check 2 - look for a thermal aureole. Search the schist within a few tens of metres of the granite for hornfels textures: spotted, chiplike, dark rock with biotite or andalusite porphyroblasts, sharp random fabric cutting the regional schistosity, and recrystallised pillow margins. Only an intrusion bakes its walls; an erosion surface cannot metamorphose the rock beneath it.
Either result alone would be persuasive; both together close the case. If a chilled margin and a 20–30 m aureole are present, no amount of regional generalisation about 'basement granite' can rescue the unconformity interpretation.
3. Should the pillow and grading evidence be allowed to decide the facing? State how you would record it.
Read the worked answer
Yes for the pillows, no for the contact - keep the two questions separate.
The granite contact tells us nothing about way-up. Facing must come only from the supracrustal rock itself. Here two independent criteria agree: pillow selvedges (thin, fine-grained, glassy-looking chilled rims, convex side up) and graded bedding fining eastwards. Two independent indicators agreeing is strong; a single one is not.
Recording:
- Enter bedding/schistosity dip-and-strike and a separate facing/younging symbol on the map - never merge them into one arrow.
- Note that the pillows are tectonically flattened; check whether the long axis lies in the schistosity plane, and use the selvedge, not the apparent shape, to read the top.
- Confirm no later folding has overturned the succession between traverses; repeat facing measurements at three or four localities along strike.
If facing is consistently east, the western part of the belt is older, and the granite contact is simply a later intrusion superimposed on a completed fold-and-metamorphism history.