U-Pb GEOCHRONOLOGICAL AND GEOCHEMICAL EVOLUTION OF DHAULADHAR AND DALHOUSIE GRANITES OF HIMACHAL HIMALAYAS

Abstract

The present form of the Earth is evolved as a result of many geological processes and events, which constantly reminding us about the dynamic nature of the Earth. Placing these geological events in their sequential and numerical order of time is a great quantitative tool in understanding and reconstructing the history of the Earth. One of the these greatest and relatively youngest geological events that occurred in the Earth’s history is the formation of the Himalayas by the collision of northward-moving Indian Plate with the Eurasian plate about no later than 57 Ma. The Himalayas is a unique example of a continent-continent collision. This mighty mountain range extends from the Nanga Parbat in the west (33o 15’ N, 74o 36’ E) to the Namche Barwa (29o 37’ N; 95o 15’ E) in the east; forming a continuous arc of about 2500 km (Le Forte, 1975) The area of study incorporates Dhauladhar and Dalhousie granites of NW Himalaya. These granitic bodies were considered as the Cambro-Ordovician Lesser Himalayan Granitic body which exhumed due to Himalayan orogeny. The present study mainly aims at U-Pb in –situ geochronology along with major, trace and rare earth element geochemistry. For this purpose, four different sampling sites were chosen and samples were collected for further processing and geochemical analysis. All 26 samples were analyzed for geochemistry and 3 samples from Dhauladhar area were collected for in-situ MC-LA-ICP-MS analysis to achieve the objectives The Dhauladhar granites are mostly coarse to medium-grained porphyritic, variably mylonatized and biotite bearing. The mineral assemblage of these granites is K-feldspar, plagioclase, and biotite. Dhauladhar granite being richer in plagioclase and biotite than the Dalhousie granites. The whole-rock geochemical studies show that Dhauladhar granites have a granodioritic to monzonitic composition with peraluminous in nature, whereas, the Dalhousie granites are having a granitic composition with highly peraluminous in nature. Both the granites show the calc-alkaline affinity as well. The Dalhousie granites are richer in, U, Th, and LREE, yet extremely depleted in Sr, Ba, Nb. They have flatter REE patterns with comparatively strong Eu anomaly (Eu/Eu*= 0.02-0.04). However, the Eu/Eu* value indicates that plagioclase abundance is greater in Dhauladhar granites than in Dalhousie granites. The major oxide composition by Harker’s variation diagram generally shows scattering with coherent trends of oxide enrichment and depletion from Dhauladhar to Dalhousie. The plots ii show a systematic increase in Al2O3, K2O, and P2O5 and decrease in CaO, Fe2O3, MgO, TiO2, MnO with increasing SiO2. The positive trend of K2O along with the moderate positive correlation of Na2O+ K2O with SiO2. Trace elements variations trends also show coherent trends of increasing peraluminicity from Dhauladhar to Dalhousie granites. On primitive mantle normalized multi-element spider diagram, all the samples show similar trace elemental distribution patterns, i.e Enrichment of LILE (Cs Rb, Th, U) to HFSE (Nb, Ti). High Rb, Th with respect to Nb with large negative Ba anomaly. The Rb/Ba vs Rb/Sr and CaO/Na2O vs Al2O3/TiO2 ratios indicate sedimentary source with the psammitic nature for Dhauladhar and pelitic nature for Dalhousie granites. The higher Rb/Sr ratios and low Sr and Ba content with low zircon saturated temperature (Tzr 603.70oC-707.30oC), Dalhousie granites support muscovite dehydration melting. While, in Dhauladhar granites, high zircon saturated temperature (Tzr 738.30o C-850.90o C) with higher Sr, Ba and low Rb/Sr ratios with negligible or no Eu anomaly suggest water flux melting. The zircon saturation temperature (Tzr) in these granites indicates the lower temperature of formation than the actual temperature of the formation. Therefore, from geochemical data and melting conditions, it can be interpreted that Dhauladahr and Dalhousie granitic are both evolved granites from different protolith along with different melting conditions. Three granitic samples (DP 1/1 N 32o 10’ 5.05” – E 76o 32’ 59.40”; DP 6/6 N32o 7’ 22.97” – E 76o 30’ 35.22”; DP 12/12 N 321o 11’ 25.66” – E 76o 29’ 29.49”) were also selected from Dhauladhar Granite for geochronology. Samples used in this study were collected along the road cut side of the three locations (DP 1/1 Palampur, DP 6/6 Baijnath, DP 12/12 Baijnath) of Dhauladhar granites. Zircon grains from all the samples show a greater range in their morphology elongated to rounded shape grains. Zircons from all the samples are mostly colorless, transparent, prismatica in shape. Some grains have inclusions of other minerals along with holes and cracks which limits the no of analytical spots. The CL images reveal the majority of the zircons grains have a typical core and rim structures. Most of the cores show the dark color and unzoned along with weak sector zoning whereas the rims show fine-scale oscillatory zoning. In addition, some zircons also appear totally planer shows no zoning. The position of U-Pb analytical spots has been decided on the basis of CL images. All three Samples (DP 1/1, DP6/6 and DP12/12) clearly show Cambro-Ordovician rims growth the Neoproterozoic cores. The cores range in age from 803 Ma, 781 Ma (DP1/DP1), 1000 Ma to 829 Ma (DP6/DP6), 802 Ma,711 Ma (DP12/12), whereas rim data show iii 480±5.5 Ma (n=7, DP1/1), 491±17 Ma (n=7,DP6/6) and 477±5.5 Ma (n=6,DP12/12). The difference between core and rim along with sharp core edges as revealed by CL images and high Th/U ratios (>0.1) indicates the igneous nature of inherited zircon characters. This clearly indicates that these bodies experienced two thermal events, first during Neoproterozoic (1200 Ma to 700 Ma) followed by Cambro- Ordovician (490-440 Ma).