karmarkar et al is a research paper on giant plagioclase basalts

1. Karmarkar et al.,1971
Giant Phenocryst Basalts in the
Deccan Trap

2. Introduction
• The porphyritic texture is very common in the
Deccan trap basalt of western Maharashtra.
• The phenocrysts in these common porphyritic
basalts are rarely larger than 1cm.
• Bcz of the characterstic pitted appearance of
Phenocryst…these are canspicuous in the
field.

3. GPB=Giant Phenocryst/Plagioclase
Basalt
• The average grainsize of groundmass
constitute is only 0.3mm.
• But in GPB the phenocrysts size is 3-6cm that
means the phenocrysts are giant 300-400
times larger than groundmass.
• So the basalts contain theses giants are called
GPB=Giant Phenocryst/Plagioclase Basalt

4. Plagioclase size and spread
• Large Phenocrysts of Plagioclase :
• 3-5cm length or
• May be as large as 10-12cm
• Found around more than 20 localities all over
western Maharashtra.
• Occurred b/w level: -30m (below mean sea
level) to 1220m (above msl)

5. • Lateral extent: >16Km
• Vertical extent: 12-45m

6. Sites
• Below 30m msl: In an exploratory drill hole at
Barvi Dam site near Badlapur.
• At 1220 m height: At Purandar, Vertical extent
here is :45m

7. Plagioclase Characterstics
• Cross-Twinning
• Phenocrysts also occur in clusters.
• Having a very characteristic pitted appearance
bcz plagioclase weathers more quickly than
ground mass.
•

8. First reported GPB
• Sowani and Peshwa (1964) described the flow
from Purandar Fort.
• Agashe and Gupte (1968) described GPBs
from eight widely separated localities.