Combined gravity and magnetic modeling over Pavagadh and Phenaimata igneous complexes, Gujarat, India: Inference on emplacement history of Deccan volcanism

Abstract A large number of igneous intrusions related to the Deccan magmatism are exposed in the western and central part of the Indian shield. Gravity and magnetic (G–M) surveys over some of these igneous intrusive bodies depict gravity high and bipolar magnetic anomalies as the most characteristic signatures. The present G–M survey was carried out over the Pavagadh and Phenaimata igneous intrusives. Associated with the Phenaimata complex, Bouguer gravity anomaly shows an elliptical shaped relative gravity high of about 40 mGal and bipolar magnetic anomaly varies from South to North between −800 nT and 1200 nT. The joint G–M modeling reveals the presence of a dense mafic body (2.86 g/cm 3 ). This body is characterized by a remanant magnetization; the related inclination ( I ) = ∼44° and declination ( D ) = ∼160° may correspond with the 29R polarity chron of Deccan magnetostratigraphy. Remanant magnetization together with age data suggest that the Phenaimata igneous intrusive emplaced during the end of the main magmatism phase of Deccan. Over the Pavagadh, a circular gravity and magnetic lows of about −15 mGal and −500 nT respectively is reported for the first time which is surrounded by a gravity and magnetic high of about 30 mGal and 350 nT, respectively. The joint G–M modeling over the Pavagadh intrusive reveals the presence of a deep-seated cone shaped high-density (⩾3.0 g/cm 3 ) gabbroic body which might extend up to a great depth. Its top surface reaches up to a depth of about 10.0 km. Overlying this body is a low-density (2.40 g/cm 3 ) rhyolite, which extends up to the surface and is the source for low gravity anomaly. It is surrounded by another high-density (2.89 and 3.02 g/cm 3 ) mafic bodies with reverse remanant magnetization direction ( I  = ∼38° and D  = ∼152°). The modeled direction of remanant magnetization for the rhyolite ( I  = −32° and D  = 336°) and deeper gabbroic ( I  = −32° and D  = 340°) bodies show normal polarity. Measured magnetization direction for the mafic body surrounding the rhyolite relates to the middle reverse polarity (29R) chron. Inferred declination and inclination may then correspond to upper normal (29N), middle reverse (29R) and lower normal (30N) polarity chrons. Therefore, the magma forming the Pavagadh igneous complex was emplaced covering the major span of Deccan eruption. G–M model suggests that the magma chambers developed within the higher crustal levels and rhyolite originated from the underlying mafic magma through assimilation and fractional crystallization (AFC).