Petrological and Geochemical Studies of Lepidolite (LCT Type) and Non-Lepidolite Pegmatite’s from Chakrasila, Dhubri District, Assam, North East India

Lepidolite pegmatite occurs as intrusive within biotite gneiss and amphibolite of Assam Meghalaya Gneissic Complex (AMGC) or Precambrian Gneissic Complex in the Dhubri district, Assam. AMGC is the north western extension of the Proterozoic rocks of Meghalaya Plateau or Shillong plateau. In the field it occurs as small to large veins and scattered boulders. Lepidolite pegmatite is later intruded by non lepidolite pegmatite. Pegmatites are medium to coarse grained with quartz and K-feldspar. It also contains lepidolite, which occurs in the form of flakes and clusters varying from pink to purple in colour. Petrography of lepidolite pegmatite reveals lepidolite as major constituents with quartz, K-feldspar and muscovite as minor constituents. XRD analysis reveals lepidolite (muscovite) is major mineral phase with kaliophilite in minor amount. Geochemically, they are calc-alkaline to high calc-alkaline and per-aluminous in nature. On the basis of Alumina Saturation Index (ASI), these pegmatites resemble Lithium-Cesium-Tantalum (LCT) family and compositional affinity with S-type granites of orogenic environments. Trace element compositions (Rb, Sr, Ba) indicate crystal fractionations, variable degrees of fractionation, highly evolved nature of pegmatite’s and strongly differentiated granites protoliths as source. The different tectonic discrimination diagrams indicate S-type and I-type melt for pegmatite derivations. Therefore, both the studied pegmatites could be an evolved variety of granitic rocks that originated from the same magma. The REE is relatively low to How to cite this paper: Meshram, R.R., Singh, B., Mishra, M.K., Hrushikesh, H., Siddiqui, A., Shukla, D., Akhtar, R. and Meshram, T.M. (2021) Petrological and Geochemical Studies of Lepidolite (LCT Type) and Non-Lepidolite Pegmatite’s from Chakrasila, Dhubri District, Assam, North East India. Open Journal of Geology, 11, 81-104. https://doi.org/10.4236/ojg.2021.113006 Received: January 21, 2021 Accepted: March 27, 2021 Published: March 30, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access R. R. Meshram et al. DOI: 10.4236/ojg.2021.113006 82 Open Journal of Geology moderate.

Granitic pegmatites represent very unusual magmas, which constitute considerable reservoirs of rare elements [8] [9]. However, the genesis and mineralization of Li-Cs-Ta (LCT) pegmatites [11] are still being debated. Processes leading to the genesis of pegmatite are defined at two distinct geological scales: 1) a crustal scale where the pegmatite-forming melt is produced; and 2) the scale of the pegmatite body, where internal physico-chemical processes lead to localized concentrations of rare elements such as Li, Be, Cs, and Ta. Most LCT-type pegmatites are interpreted as the product of extreme granitic fractionation. Such a magmatic process is defined by fractional crystallization leading to an increase of rare elements and fluxes in the residual melt with increasing distance from the consolidating parental granitic source (see [1] [12] [13] [14], and references cited therein).
Lepidolite occurrence from the area was first reported by [35] and carried out detailed investigation and traced lepidolite bearing boulders over a 120 m × 30 m zone [36]. Subsequently, preliminary work carried out at Chakrasila and adjoining area through collection of geochemical samples and drilling [37] [38] [39]. The work for W, Sn & REE in Ckakrasila area has been attended and indicated lepidolite occurrence in pegmatite with higher concentration of Rb and Y [40]. The purpose of this paper is geochemistry and genesis of lepidolite and non-lepidolite pegmatites.

Geological Background
The Assam Meghalaya Gneissic Complex (AMGC) is the north eastern extension of Indian Shield and is separated from the main mass of Peninsular India by Tertiary sediments of Ganges Brahmaputra and Cretaceous Rajmahal volcanics. The Shillong plateau composed of AMGC is considered as the detached portion of Eastern Ghats Mobile belt [41] or Chotanagpur Gneissic Complex [42] ( Figure  1, after [43] [44] [51]. Shillong Group has undergone green schist facies of metamorphism [52]. Proterozoic-Early Paleozoic (881 -479 Ma) equigranular to porphyritic coarse grained granite-granodiorites plutons (Mylliem granites and its equivalents) intruding the basement gneisses and the Shillong Group of rocks [49] [53]- [58]. Basic volcanics of Sylhet /Mikir traps of Cretaceous age occurs as concordant to discordant bodies within the Shillong group of rocks. The plateau is bounded and dissected by various fault systems E-W and N-S to the major earthquakes in the region. To North the plateau is bounded by Oldham fault [59] and to south it is flanked by Dauki fault [60] also considered as the extension of Son Narmada Fault. Advanced studies incorporated with chemical dating suggests that in the global scenario AMGC can be probably a leading edge during oblique collision between India with Austarlo Antartica during Pan-African final amalgamation and the Pan-African suture passing through Prydz Bay in Antarctica continued to the Shilling plateau which passes in between Sonapahar and Garo-Goalpara Hills regions of the Shillong plateau [48].
The study area is the part of Assam Meghalaya Gneissic Complex (AMGC)/ Precambrian Gneissic Complex in the Dhubri district, Assam. In Assam, the rocks of AMGC are the north western extension of the Proterozoic rocks of Meghalaya Plateau or Shillong plateau [59] [61]. The Precambrian rocks of AMGC are dominated by granulite to amphibolite facies of rocks interlayered with Banded Iron Formations (BIF), amphibolites, talc-actinolite schist and pyroxenites [40]. Gneissic complex occupies a large part of the central Assam and few isolated inselbergs cutting out of the Quaternary plains of western Brahmaputra Basins. The Gneissic Complex comprises of biotite-bearing quartzo-feld-spathic gneiss, schist, biotite-hornblende gneisses, migmatitic granitoid intruded by younger acidic (granite, aplite, pegmatite) and basic (metadolerite, epidiorite, amphibolite) intrusive rocks ( Figure 1 (A), after [40]).
1) Lepidolite pegmatite: Lepidolite occurred as flakes within pegmatite veins, which is intruded in biotite gneiss and amphibolite in the northern tip of Dhir Bill (at Chakrasila). Amphibolite and actinolite tremolite schist are associated rocks in the area and are in the form of small bodies and boulders on Chakrasila hillock. In situ lepidolite/pegmatite veins as exposed in the area, only large blocks and boulders of lepidolite rocks scattered over the surface (Figure 2(A) and Figure 2(B)). The large blocks and boulders of lepidolite bearing pegmatite spread over the surface for a length of 200 m with a width of 120 -130 m. It occurs in the form of fragmented boulder rather than continuous body/exposure. At places these boulders are traversed by quartz veins of 30 -40 cm length and width of 2 -4 cm. The lepidolite predominantly pink to purplish-violet and in the form of fine to coarse flakes, radiating clusters with subordinate quartz, feldspar and muscovite ( Figure  2(C)). From the nature of concentration of blocks and boulders of lepidolite pegmatite over gently sloping ground it seems that they are in situ. Pegmatite is medium to coarse grained and is composed of lepidolite, quartz, alkali feldspar, muscovite and opaque minerals. It is massive, hard and compact.
In petrography, pegmatite is medium to coarse grained and is composed of lepidolite, quartz, alkali feldspar, muscovite and opaque minerals. Lepidolite is in the form of tabular to platy, laths to euhedral crystals, medium to coarse In thin section, it is medium to coarse grained and consist of quartz, K-feldspar and plagioclase, biotite and muscovite ( Figure 2(H)). Garnet, zircon and opaque minerals occur as accessory mineral. The quartz grains are mostly subhedral to anhedral with deformation and shows wavy extinction and at places play undulose extinction. Biotite and muscovite is lath shaped and is present as very thin flakes ( Figure 2(I)). Biotite is dark brown to dark yellowish and shows pleochroic halos surrounding the zircon grain ( Figure 2(J)). K-feldspar is subhedral and more dominant over plagioclase-feldspar. At some places, microcline with cross hatched twinning is observed. Plagioclase feldspars occur as subhedral grains with polysynthetic twinning. Some overgrowth texture also noticed, K-

Geochemistry of the Pegmatites
The whole-rock major-and trace-element compositions for pegmatites are presented in Table 2 and Table 3. The different geochemical variation and tectonic diagrams are plotted using these data.
1) Major oxide Geochemistry In lepidolite pegmatite SiO 2 is high ranging from 51.43 to 77.07 wt% (except one sample having 49.93 wt% might be due to contamination of mafic phase or error in sample collection). High Al 2 O 3 varies from 12.13 to 28.65 wt%, low to high K 2 O upto 8.99 wt%, Na 2 O from 0.19 to 2.74 wt%. Low amount of CaO ranging from 0.01 to 1.12 wt% (except one sample 10.82 wt%) and P 2 O 5 from 0.01 to 9.59 wt% (high in one sample). Low amount of MnO varying from 0.01 to 1.25 wt%, MgO from 0.01 to1.91 wt% (except 5.92% MgO in one sample) and     [62], the compositions of samples plotted closed to the field of alkali granite and granite with sub-alkaline to alkaline nature (Figure 4(A)). The SiO 2 vs K 2 O binary diagram (after, [63]) indicates calc-alkaline to high calc-alkaline characters for majority samples. Whereas, some samples plotted in tholeiitic field and few lepidolite samples in soshonitic field (Figure 4(B)). The molar Al 2 O 3 /(CaO + Na 2 O + K 2 O) versus molar Al 2 O 3 / (Na 2 O + K 2 O) (A/CNK vs A/NK) diagram [64] [65] reflect the per-aluminous character of both the pegmatites (Figure 4(C)). In SiO 2 vs Na 2 O+K 2 O-CaO (MALI) diagram of [66] samples plot in the field of calcic and calc-alkalic field. The overlapping field of I-type, A-type and S-type are from [67], where studied samples indicate both S-type and I-type magmatic characters (Figure 4(D)).
2) Trace and immobile element geochemistry In lepidolite pegmatite, trace element Ba varies from 50 to 821 ppm, Ga from 26 to 163 ppm and Rb from 239 to 393 ppm (three samples have 12,409, 13,138 & 15,537 ppm Rb content). Sr from 6 to 130 ppm, Y ranging from 21 to 939 ppm, Zn from 10 to 676 ppm. In non lepidolite pegmatite, trace element Ba varies from 50 to 1241 ppm, Ga from 5 to 41 ppm and Rb from 3 to 1268 ppm. Sr from 5 to 186 ppm, Y ranging from 5 to 91 ppm, Pb from 12 to 319 ppm. a) Tectonic discriminations and fractional crystallization: Trace element compositions of the studied rocks are presented, briefly. An increase in the amounts of Rb can be correlated with late stage crystallization of K-feldspar and biotite, and decrease in Sr contents can be due to fractional crystallization of plagioclase. Barium content is commonly used as indicator of the evolution in granites and pegmatites, which decreases with increasing crystal fractionation [73] [74], and the decrease in content of Ba is marked. High amount of Ba (50 -821 ppm) is due to the presence of minerals, such as K-feldspar and biotite. Rubidium contents in the pegmatites vary from 239 to 393 ppm and Sr contents from 6 to 130 ppm. With increasing concentrations of K, the Rb contents of the rocks increase, as well. The increasing concentration of Ba, Sr and Cs values, are also observed in samples due to their similar geochemical behavior. These chemical characteristics indicate that the studied pegmatites are highly evolved varieties of granitoids of the region. As a rare alkali metal Rb is enriched in K-bearing minerals during progress of pegmatite crystallization [73], the ratio of K/Rb is indicative of the general fractionation.
A hybridization index of CaO + MgO + FeO T is useful in quantifying deviations from leucogranitic melt compositions (see [75]). Rb vs. Sr and Rb vs. Ba plots showing variable degrees of fractionation within the simple-type and hybridized pegmatites and granites; the trend for normal granites is plotted for comparison. The pegmatites are strongly to moderately evolved, as demonstrated by Rb-Ba-Sr trends that vary between signatures of a normal and a moderately fractionated granitic pegmatite [76]. The majority of the pegmatites plots away the normal granite trend. Ratios of K/Rb vs. Cs (see [73]) are good proxies to evaluate the K-Rb and K-Cs substitution in potassium feldspar and micas within the bulk samples. The granites differ from the granitic pegmatites by having significantly lower Rb, but comparable Cs contents. The studied samples follow a simple type pegmatite fractionation pathway on the K 2 O/Rb vs. Rb plots. The trends are very steep, increasing sharply in Rb. The pegmatites clearly lie on a deeper K 2 O/Rb vs. Rb path.
b) The pegmatite protoliths (source rock) characteristics: A few samples contain progressively less Sr and Ba and more Rb as a result of fractionation. High amounts of Rb in the studied pegmatites, indicate that these samples are placed in the category of strongly differentiated granites in the ternary Rb-Ba-Sr plot [76]. Other samples are moderately evolved chemically. On the basis of diagram from [77] studied pegmatites plot mainly in S-type granites fields and few samples plot in I-type field indicate involvement S-type granite as major source along with I-type granite as minor constituents. Therefore, both the studied pegmatites could be an evolved variety of granitic rocks that originated from the same magma.
The Rb vs Sr and Rb vs Ba (ppm) diagrams showing variable degrees of fractionation, the studied samples plotted above the normal granite field ( Figure  5(A) and Figure 5(B)). The trend of normal granite field is after [76]. Three samples have more than 10,000 ppm Rb and not plotted in the diagram, which indicate more fractionation of Rb, Sr and Ba in both the pegmatites. Rb vs K 2 O/Rb (ppm) diagram (after, [78]) evaluate the K-Rb fractionation in simpletype and complex-type (hybridized) pegmatites. The analysed data plot near the arrow of simple type pegmatite and few samples scattered around hybrid pegmatite ( Figure 5(C)). The trace element Zr vs TiO 2 diagram [79] indicate mainly S-type granitic source for both the pegmatites and few samples of plots in I-type field ( Figure 5(D)). In triangular CaO-Al 2 O 3 -Na 2 O-K 2 O-Fe 2 O 3T + MgO diagram (after, [77]) indicate mainly S-type field and I-type field for some samples ( Figure 5(E)) and Zr vs Nb/Ta (ppm) diagram (after, [78]) indicate enrichment and fractionation of Nb ( Figure 5(F)). The Nb/Ta ratios for pegmatites range between 0.31 and 30.93 in lepidolite pegmatite and between 4.11 and 61.98. Nb/Ta vs. Ta and Nb/Ta vs. K 2 O/Rb plots show variations in Nb/Ta indicate fractionation between simple-type pegmatites vs. hybridized pegmatites; the arrows indicate the direction of a more evolved (i.e. fractionated) melt undergoing hybridization. In Nb/Ta vs Ta samples plot in lower end of hybridization arrow and few towards the arrow head ( Figure 5(G)) and Nb/Ta vs K 2 O/Rb diagram samples plot around hybridization arrow indicate more fractionation of Ta and Rb in lepidolite pegmatite ( Figure 5(H)). Variations in Nb/Ta and Zr/Hf also infer a moderate to high degree of fractionation in the pegmatite melts. Similarly, the immobile elemental magmatic affinity plot of [80] in Yb vs Th (ppm) diagram data indicate calc-alkaline affinity ( Figure 5(I)) and in Yb vs La (ppm) samples plot in the tholeiitic to transitional field and samples of lepidolite pegmatite in Open Journal of Geology  [78]), (D) The trace element Zr vs TiO2 diagram indicate mainly S-type granitic source for both the pegmatites (after, [79]), (E) In triangular CaO-Al2O3-Na2O-K2O-Fe2O3T + MgO diagram indicate mainly S-type field and I-type field for some samples (after, [77]), (F) Zr vs Nb/Ta (ppm) diagram indicate enrichment and fractionation of Nb (after, [78]), (G) In Nb/Ta vs Ta samples plot in lower end of hybridization arrow and few towards the arrow head, (H) Nb/Ta vs K2O/Rb diagram samples plot around hybridization arrow indicate more fractionation of Ta and Rb in lepidolite pegmatite, (I) Yb vs Th (ppm) diagram data indicate calc-alkaline affinity in immobile elemental magmatic affinity plot [80], (J) Yb vs La (ppm) samples plot in the tholeiitic to transitional field and samples of lepidolite pegmatite in transitional to calc-alkaline field, (K) Ternary Hf-Rb/30-Ta*3 diagram reflects mainly within plate tectonic field, and for some samples all other fields [81] and (L) Ba-Rb-Sr ternary diagram represent normal to strongly differentiated trend of granite/pegmatite (after, [76]). Symbols are same as used in Figure 4. transitional to calc-alkaline field ( Figure 5(J)). Ternary Hf-Rb/30-Ta*3 diagram [81] samples reflect within plate and volcanic arc tectonic fields, and for some samples all other fields ( Figure 5(K)) and Ba-Rb-Sr ternary diagram (after, [75]) represent normal to strongly differentiated trend of granite/pegmatite with few samples as anomalous granite (Figure 5(L)).
3) Rare Earth Element geochemistry The Rare Earth Element (REE) data of both the pegmatites is presented in Table 4. The REE spider and multi-element spider variation diagrams were plotted using these data.    Figure 6(B)) and multi element spider diagram ( Figure   6(C) & Figure 6(D)) after [82] represented following observations. The chon- vation of such elements in residual phases when magmas have been generated in a subduction zone by partial melting of source rocks (see [84]). Positive anomaly of Rb in multi-elements spider diagram may have resulted from late stage crystallization of muscovite and K-feldspar from magma. Barium, and Sr negative anomalies can be due to their co-substitution in plagioclase, which crystallizes at early stages.

Rare Earth
On the whole, enrichment in some LILE, such as K, Rb, and Th and depletion in some HFSE, such as Nb, Ti, Zr, and Y, and HREE can be related to melting and fractionation processes in the region [85] [86]. According to the LCT (Li-Cs-Ta) family of pegmatites contains high concentrations of Rb, Cs, Be, Ta, Nb, and Sn, as well as elevated levels of fluxing components (Li, P, F, and B). Accordingly, the studied lepidolite pegmatites have high concentration of Rb (up to 393 ppm; three samples have 12,409, 13,138 & 15,537 ppm), Ta (up to 269.86 ppm), Nb (up to 83 ppm), and Sn (312.85 ppm). High amounts [87] of HFSE elements, such as Th (up to 102 ppm), U (up to 11.26 ppm), and Zr (up to 259 ppm) are may be due to occurrence of some minerals, such as Th-silicate, U-silicate, U-oxides, and zircon in pegmatites. Accordingly, the studied non lepidolite pegmatites have high concentration of Rb (up to 1268 ppm), Nb (up to 46 ppm), HFSE element such as Th (up to 42 ppm), U (up to 93.36 ppm), and Zr (up to 493 ppm) are might be due to occurrence of some minerals, such as Th-silicate, U-silicate, U-oxides, and zircon in pegmatites. These above REE studies emphasize the highly fractionated nature of both these pegmatites from the granitic source.

Conclusion
On