Two-Stage, Extreme Albitization of A-type Granites from Rajasthan, NW India

Albitization is a common process during which hydrothermal fluids convert plagioclase and/or K-feldspar into nearly pure albite; however, its specific mechanism in granitoids is not well understood. The c. 1700 Ma A-type metaluminous ferroan granites in the Khetri complex of Rajasthan, NW India, have been albitized to a large extent by two metasomatic fronts, an initial transformation of oligoclase to nearly pure albite and a subsequent replacement of microcline by albite, with sharp contacts between the microcline-bearing and microcline-free zones. Albitization has bleached the original pinkish grey granite and turned it white. The mineralogical changes include transformation of oligoclase (∼An12) and microcline (∼Or95) to almost pure albite (∼An0·5-2), amphibole from potassian ferropargasite (XFe 0·84-0·86) to potassic hastingsite (XFe 0·88-0·97) and actinolite (XFe 0·32-0·67), and biotite from annite (XFe 0·71-0·74) to annite (XFe 0·90-0·91). Whole-rock isocon diagrams show that, during albitization, the granites experienced major hydration, slight gain in Si and major gain in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn, light rare earth elements and U. Whole-rock Sm-Nd isotope data plot on an apparent isochron of 1419 ± 98 Ma and reveal significant disturbance and at least partial resetting of the intrusion age. Severe scatter in the whole-rock Rb-Sr isochron plot reflects the extreme Rb loss in the completely albitized samples, effectively freezing 87Sr/86Sr ratios in the albite granites at very high values (0·725-0·735). This indicates either infiltration of highly radiogenic Sr from the country rock or, more likely, radiogenic ingrowth during a considerable time lag (estimated to be at least 300 Myr) between original intrusion and albitization. The albitization took place at ∼350-400°C. It was caused by the infiltration of an ascending hydrothermal fluid that had acquired high Na/K and Na/Ca ratios during migration through metamorphic rocks at even lower temperatures in the periphery of the plutons. Oxygen isotope ratios increase from δ18O = 7‰ in the original granite to values of 9-10‰ in completely albitized samples, suggesting that the fluid had equilibrated with surrounding metamorphosed crust. A metasomatic model, using chromatographic theory of fluid infiltration, explains the process for generating the observed zonation in terms of a leading metasomatic front where oligoclase of the original granite is converted to albite, and a second, trailing front where microcline is also converted to albite. The temperature gradients driving the fluid infiltration may have been produced by the high heat production of the granites themselves. The confinement of the albitized granites along the NE-SW-trending Khetri lineament and the pervasive nature of the albitization suggest that the albitizing fluids possibly originated during reactivation of the lineament. More generally, steady-state temperature gradients induced by the high internal heat production of A-type granites may provide the driving force for similar metasomatic and ore-forming processes in other highly enriched granitoid bodie

New evidence for two sharp replacement fronts during albitization of granitoids from northern Aravalli orogen, northwest India

<div><p>We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink microcline–albite granite, and stage II converted the microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, biotite (annite-rich) and hastingsite (amphibole) to hastingsite with low X_Fe values, dehydration, gain in Na, and losses in Fe and Rb. The second stage of metasomatism caused almost complete conversion of microcline to albite and complete or nearly complete disappearance of amphibole. Chemically, these changes are manifested by substantial gain in Na and extreme losses in K, Rb, Ba, Ca, Sr, Fe, and Mg. Depending on the modal abundances of amphibole, stage II albitized rocks are depleted in light rare earth elements or heavy rare earth elements or both, signifying that rare earth elements are principally hosted by mafic phases. The disparity in whole-rock δ¹⁸O values during both stages of albitization is related to the variations in modal amounts of Si-bearing phases. The replacement microstructures are in accord with the fluid-mediated phase transformations by a coupled dissolution–precipitation mechanism. The albitizing event took place at low temperatures of 350–400 °C and the fluid was metamorphic in nature.</p></div