Geochemistry and in-situ U-Th/Pb Geochronology of the Jambil Meta-Carbonatites, Northern Pakistan: Implications on Petrogenesis and Tectonic Evolution

0. INTRODUCTION

Carbonatites are important due to their economic significance, great potential of REE and rare metals, such as Nb, Th, V, Sc and Ta etc. Carbonatites, are igneous rocks with more than 50 modal percent calcite (CaCO₃), dolomite [CaMg(CO₃)₂] and/or siderite (FeCO₃) with silica (SiO₂) content usually less than 20 wt.% (Le Maitre et al., 2005). Metamorphosed carbonatites (meta-carbonatites) can be difficult to distinguish from marble in the field, particularly when both are exposed to amphibolite and higher grades of metamorphism. Initial intrusive relationships and textural signatures may be overprinted during a subsequence of deformation and metamorphism, leaving both with a similar granoblastic texture (Cherbal et al., 2019; Lastochkin et al., 2011; Scogings and Forster, 1989). Carbonatites are generally thought to be mantle-derived magmatic rocks emplaced within extensional environments in a stable craton (Evans, 2009), however, some carbonatites may have evolved in a compressional tectonic environment, e.g., the Qinling in China (Ying et al., 2017; Xu et al., 2015, 2014, 2007) and Dunkeldik in Pamir orogenic belt, Tajikistan (Hong et al., 2019). Primary carbonatitic melt can be produced by partial melting of either carbonated peridotite or carbonated eclogite, where the source of carbon may be primordial mantle or recycled oceanic carbonate, respectively (Xu et al., 2014; Manthilake et al., 2008; Ying et al., 2004; Hammouda, 2003; Hoernle et al., 2002; Bell and Tilton, 2001).

Prior to the Cenozoic Himalayan orogeny, the northern margin of the Indian Plate was part of the northern Gondwana supercontinent during Early Paleozoic time (Wang et al., 2020, 2019; Sajid et al., 2018; Faisal et al., 2016; Cawood et al., 2007), which has experienced several extensional/rifting and convergent tectonic episodes during its geological history. Research on the northern margin of the Indian Plate has commonly focused on the Cenozoic Himalayan orogeny and associated tectonism, magmatism and metamorphism; however, the pre-Himalayan events are relatively rare and commonly overprinted by pervasive and extreme Tertiary Himalayan tectonics. The presence of Early Paleozoic Kafiristan, Utla, and Mansehra plutons from north Pakistan contain evidence of an Early Paleozoic extensional tectonic episode (Sajid et al., 2018; Faisal et al., 2016). It is, thus, surprising that neither alkali-rich silica-undersaturated nor carbonatitic rocks, commonly associated with extensional tectonics, had been reported from the northern Indian Plate margin. The occurrence of Jambil meta-carbonatites in the Proterozoic Mangluar Formation in the Lower Swat Valley, North Pakistan (Fig. 1) is an important step forward in the study and interpretation of the geological history of the northern Indian Plate margin in Northwest Pakistan. This is particularly so because the Jambil meta-carbonatites occur in an unusual petrographic milieu lacking undersaturated silicate rocks commonly found in spatial association with carbonatites elsewhere in the world. This locality is also the first confirmed occurrence of carbonatites in the northern Indian Plate margin and interpretation of the data gleaned from the Jambil meta-carbonatites is important in unraveling the geodynamic evolution of the northern Indian Plate margin. A single apatite fission track age of 29.3 ± 1.2 Ma has been published from the carbonatites, which is interpreted as its emplacement age (Khattak et al., 2004). In the present contribution we provide new geochemical and geochronological data (U-Th/Pb titanite and monazite) from the Jambil meta-carbonatites to enhance our understanding on their petrogenesis, crystallization and tectonic significance.

Figure  1.  Regional geological maps of (a) NW Pakistan and (b) the Jambil-Mingora area (after DiPietro, 1990), showing location of the carbonatite bodies numbered as: 1. a meter-thick sill of white carbonatite, 2. mottled brown carbonatite and 3. assumed carbonatite body of Butt and Shah (1985) and Khattak et al. (2004); demonstrated as a calc-silicate marble hereafter.

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1. GEOLOGICAL SETTING

Northern Pakistan is underlain by rocks of the Indian Plate, Kohistan Island Arc and Asian Plate separated by two regional thrust zones: the main mantle thrust (MMT) and main Karakorum thrust (Faisal et al., 2016, 2014; Burg, 2011; Rehman et al., 2011; Searle and Treloar, 2010). The Jambil meta-carbonatites occur within Indian Plate rocks ~10 km southeast of Mingora in the Lower Swat Valley (Figs. 1a–1b). The actual extent of these intrusive bodies is unknown because a thick soil layer covers much of the area (Fig. 2). The small isolated carbonatite bodies that are exposed intrude the Proterozoic Manglaur Formation, which is also intruded by the Permian Swat granitic gneiss (Sheikh et al., 2020; Anczkiewicz et al., 2001; DiPietro and Isachsen, 2001). South of the Swat Valley, the Swat granitic gneiss and Manglaur Formation can be correlated with the Permian Ambela granitic complex in the Peshawar Basin and Proterozoic Tanawal and Hazara formations in the Abbottabad area, respectively. The Manglaur Formation includes strained garnet mica schist, quartz muscovite schist, quartzite, calcite marble, tremolite marble, feldspathic quartzite and minor graphitic schist all of which have been affected by multiple folding events (DiPietro and Lawrence, 1991; DiPietro, 1991, 1990). The Swat granitic gneiss, and locally the Manglaur Formation itself, is unconformably overlain by the dominantly calcareous metasedimentary succession of the Alpurai Group (Pogue et al., 1992; DiPietro, 1990). The Alpurai Group can be subdivided into a Carboniferous rift facies of the Marghazar Formation and a Triassic to Jurassic passive carbonate shelf deposits of Kashala, Nikanai Ghar and Saidu formations (DiPietro et al., 1993). However, in the Peshawar Basin and Abbottabad area the Early Paleozoic quartzites, argillites and carbonates unconformably overly either the Proterozoic (Tanawal Formation) or Cambrian sequence (Ambar Formation) (Palin et al., 2018; Qasim et al., 2018; Pogue et al., 1992). In the Swat Valley, the Alpurai Group is overridden by ophiolitic mélange of the MMT zone as a result of Paleocene–Eocene Himalayan orogenesis (DiPietro et al., 1993).

Figure  2.  Outcrop photos of the Jambil meta-carbonatites (a)–(b) intruding the Proterozoic Mangluar Formation and calc-silicate marble (c)–(d) of the Mangluar Formation.

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The appearance of the Jambil meta-carbonatites varies with location (Fig. 1b), including at location 1 a meter thick, sill-like white and sugary textured carbonatite, more akin to marble, and location 2 mottled brown carbonatite (Figs. 2a–2b). At location 3 near Meragai Village (Fig. 1b), Butt and Shah (1985), and later Khattak et al. (2004), described a grayish white carbonatite body they interpreted to intrude the Proterozoic Manglaur Formation (Figs. 2c–2d). The rocks in this exposure are strongly deformed along with associated garnetiferous schist and quartzite. However, geochemical data in the present study show that the previously described carbonatite at location 3 has a sedimentary protolith, and are therefore, referred as calc-silicate marble hereafter.

2. ANALYTICAL METHODS

The Jambil meta-carbonatites (at location 1) contain titanite and monazite, suitable for U(-Th)/Pb geochronology. This work was conducted through laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the University of British Columbia, Okanagan in Canada in polished thin sections. The LA-ICP-MS setup includes a Photon Machines Analyte 193 Excimer laser coupled to an Agilent 8900 triple quadrupole ICP-MS housed in the Fipke Laboratory for Trace Element Research (FiLTER). Helium (0.7 L/min) was used as the ablation cell carrier gas and was mixed with Argon (0.9 L/min) before the plasma using an in-house glass mixing valve that doubled as a signal smoothing device. The instrument was optimized before each analytical run using the Standard Reference Material 'NIST610' for maximum signal while maintaining a ²³⁸U/²³²Th ratio within 3% of the certified value. Dwell times of 50 ms were used for U/Th/Pb isotopes. The laser was operated with a spot size of 40 and 20 µm, repetition rate of 8 and 5 Hz for ablation of titanite and monazite, respectively, and a fluence of 5 J/cm² for both minerals. Each spot was pre-ablated with two laser bursts to clear the surface followed by 30 s delay and then 30 s of ablation. Instrument drift was corrected by bracketing samples with known standards. The titanite data were normalized to titanite reference material 'MKED-1' as a primary standard (²⁰⁶Pb/²³⁸U age of 1 517.32 ± 0.32 Ma; Spandler et al., 2016) with 'Fish Canyon' used as secondary reference material (²⁰⁶Pb/²³⁸U age of 28.2 Ma; Lanphere and Baadsgaard, 2001). Thirteen repeat analyses of 'Fish Canyon' yielded a lower intercept age of 28.3 ± 2.84 Ma (mean squared weighted deviation (MSWD) = 1.9), well within 2% (systematic uncertainty of the LA-ICP-MS U-Pb method) of the accepted value. Similarly, the U-Th/Pb data of monazite were normalized to reference material '44069' with 'Bananeira' and '554' used as secondary reference standards with ²⁰⁶Pb/²³⁸U ages of 424.9 ± 0.6 Ma (Aleinikoff et al., 2006), 511.9 ± 1.5 Ma (Palin et al., 2018) and 46.8 ± 0.7 (Kylander-Clark et al., 2013), respectively. The ²⁰⁸Pb/²³²Th ages of the monazite reference materials; 44069, Bananeira and 554 are 424.9 ± 0.6, 497.6 ± 1.6 and 46.7 ± 0.2 Ma (Kylander-Clark et al. 2013), respectively. Thirteen repeat analyses of 'Bananeira' yielded a ²⁰⁶Pb/²³⁸U age of 494 ± 3 Ma (MSWD = 2.12) and a ²⁰⁸Pb/²³²Th age of 495 ± 1.55 Ma (MSWD = 1.55). Those ages are within the systematic uncertainty of the method especially considering the variation in the U-Pb age of 'Bananeira' (Gonçalves et al., 2016). Six replicate analyses of '554' yielded a ²⁰⁶Pb/²³⁸U age of 52.3 Ma (MSWD = 1.51) and a ²⁰⁸Pb/²³²Th age of 48.0 ± 2 Ma (MSWD = 1.11). While the slightly older ²⁰⁶Pb/²³⁸U age may reflect unsupported ²⁰⁶Pb from the decay of ²³⁰Th (Schärer, 1984), the ²⁰⁸Pb/²³²Th age is indistinguishable from that expected. The ChrontouR package, open access scripts for plotting geochronological data (Larson, 2020), was used to plot concordia diagrams and calculate intercept and weighted mean ages.

Whole rock major elements were analyzed through X-ray fluorescence (XRF) on glass pellets of ground sample fluxed with lithium metaborate at the Central Analytical Facility Division, Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan. Trace elements, including REE, were determined by inductively coupled plasma mass spectrometry also at the PINSTECH by dissolving powdered sample in Teflon bombs using acid mixture.

Quantitative electron microprobe analyses were performed on polished thin sections at State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (CUG), Wuhan, using a JEOL JXA-8100 probe equipped with wavelength-dispersive spectrometers with an accelerating voltage of 15 kV, beam current of 20 nA, beam size of 5 µm and counting time of 10 s. Orthoclase (K, Al), albite (Na), diopside (Ca, Si), olivine (Mg), hematite (Fe), rhodonite (Mn), titanite (Ti), fluorite (F), celestite (Sr), barite (Ba) and Ce-phosphate (P) were used as standards to measure oxides of the elements in brackets.

Calcite C-O isotopic compositions were determined using a MAT251 isotope ratio mass spectrometer at GPMR, CUG, Wuhan. Following the procedure of McCrea (1950), calcite powder was dissolved in phosphoric acid at 25 ℃ for 12 h. CO₂ liberated by acid reaction was analyzed for its carbon-oxygen isotopic composition and results were reported in per mil variation (‰) relative to PDB (Pee Dee Belemnite) and SMOW (Standard Mean Ocean Water), respectively.

3. RESULTS

3.1 Petrography and Mineral Chemistry

In hand specimen, the Jambil meta-carbonatites (locations 1 and 2) are white to mottled brown, medium-grained with a sugary texture. Under the microscope, it consists of a mosaic of carbonates, diopside, barite, monazite, traces of epidote, titanite, apatite, opaques (pyrite), sodic plagioclase and K-feldspar (Figs. 3a–3e).

Figure  3.  Cross-polarized photomicrographs (a), (b) and back-scattered electron images (c), (d), (e) of the Jambil meta-carbonatites (locations 1 and 2 discussed in the text). (f) Cross-polarized photomicrograph of the calc-silicate marble (location 3 discussed in the text). Minerals abbrevations are: Cal. calcite; Tit. titanite; Dio. diopside; Ap. apatite; Mnz. monazite; Bar. barite.

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The calc-silicate marble (location 3) is medium-grained with a subequigranular texture in hand specimen. In thin section, it consists of carbonate minerals (calcite and minor dolomite), diopside, titanite, plagioclase, minor amounts of apatite, ilmenite, epidote and opaque minerals (Fig. 3f). Calcite and diopside are equigranular, subhedral to euhedral forming well-developed triple junctions (Fig. 3f). Titanite and trace plagioclase and apatite are subrounded, subhedral and lack zonation. Most of the opaque minerals are magnetite and pyrite with euhedral to subhedral shapes.

Carbonate mineral data of both meta-carbonatites and the calc-silicate marble are plotted on a FeO-MnO-SrO triangular diagram (Fig. 4a). Carbonates from the calc-silicate marble plot inside the field quantified by Le Bas et al. (2002) for common marble while calcite in the meta-carbonatites record a similar trend to several other known carbonatite bodies. Calcite is the only carbonate mineral in the meta-carbonatites. It is characterized by high SrO (0.54 wt.%–1.83 wt.%) and MnO (0.19 wt.%–1.24 wt.%) (Table 1; Fig. 4b). In contrast, calcite and trace dolomite in the calc-silicate marble have low SrO and MnO, consistent with that expected in common marbles (Fig. 4b). Finally, the MnO/FeO ratio of calcite from the meta-carbonatites (0.63–2.87) is higher than that the marble (0.22–0.33) (Table 1).

Table  1.  Representative composition (wt.%) of carbonates in Jambil carbonatites and calc-silicate marbles

	Carbonatites									Calc-silicate marble (Meragai)					Miaoya Carbonatite (China)a		Delbek Carbonatite (Turkestan)b		Borra Marble (India)c
	JS-46					JS-52				JS-49
	1	2	3	5		7	8	9		11	12	13	14				Av (70)		Av (5)
FeO	0.3	0.75	0.71	0.31		0.69	0.81	0.29		0.37	0.35	0.91	0.09		0.75		0.23		0.28
MnO	0.19	1.09	1.02	0.89		0.98	1.24	0.6		0.08	0.1	0.29	0.03		0.48		0.35		0.09
MgO	1.23	0.72	1.22	0.14		1.13	0.66	0.46		1.27	1.22	1.3	20.78		0.4		0.65		0.99
CaO	54.12	53.13	52.01	53.33		53.21	53.13	54.77		54.8	54.6	53.53	30.12		53.37		54.71		54.6
SrO	0.54	0.83	1.83	1.27		0.87	0.76	0.71		0.1	0.12	0.09	0.03		1.24		0.51		0.11
BaO	0.06	BD	0.01	BD		BD	BD	BD		0.22	0.13	BD	0.1		0		-		0.11
Total	56.44	56.52	56.8	55.94		56.88	56.6	56.83		56.84	56.52	56.12	51.15		56.24		56.45		56.18
MnO/FeO	0.63	1.45	1.44	2.87		1.42	1.53	2.07		0.22	0.29	0.32	0.33		0.64		1.52		0.32
Data source: a (Xu et al., 2010); b (Vrublevskii et al., 2018); c (Le Bas et al., 2002).

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Figure  4.  (a) MnO-SrO-FeO (wt.%) triangular plot of carbonate composition in the Jambil meta-carbonatites, calc-silicate marbles and in other known carbonatites and marbles (data from Vrublevskii et al., 2018; Xu et al., 2010; Le Bas et al., 2002). The carbonates of calc-silicate marbles plot within non-carbonatitic carbonate composition area outlined by Le Bas et al. (2002); (b) Plot of SrO (wt.%) versus MnO (wt.%) in carbonate minerals from Jambil meta-carbonatites and calc-silicate marble. The plot also shows comparison with other carbonatitic and sedimentary carbonates elsewhere in the world used to distinguish carbonatitic and sedimentary carbonates. The carbonates of calc-silicate marbles plot in a small box close to the origin. The box represents SrO and MnO contents of sedimentary carbonates (data taken from Yang and Le Bas, 2004; Tucker and Wright, 1990; Scoffin, 1987; Veizer, 1983; Veizer et al., 1978).

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Apatite in the meta-carbonatites are characterized by high SrO and light REE, and low Cl contents, which is similar to apatite in meta-carbonatites reported from around the world (Viladkar and Subramanian, 1995; Platt and Woolley, 1990; Hogarth, 1989; Sommerauer and Katz-Lehnert, 1985) (Table 2). Unlike those in the meta-carbonatites, apatite in the calc-silicate marbles are relatively low in SrO and REE and rich in both Cl and F contents, similar to apatite previously reported from marbles (Le Bas et al., 2002; Chang, 1998). Moreover, the Sr and Na contents of apatite generally increase with increasing metamorphism to a maximum during granulite facies (Le Bas et al., 2002). Despite that association, the Sr and Na contents of apatite in calc-silicate marbles still remain significantly lower than the meta-carbonatites. Finally, the F/Cl ratio of carbonatite apatite (134–202) is significantly higher than those in calc-silicate marble (≤4) (Table 2).

Table  2.  Representative major element (wt.%) analysis of apatite in Jambil carbonatites and calc-silicate marbles

	Carbonatites								Calc-silicate marble (Meragai)			Miaoya Carbonatite (China)a		Delbek Carbonatite (Turkestan)b		Borra Marble (India)c
	JS-46					JS-52			JS-49
	1	2	3	5		7	8		11	12				Av (70)		Av (5)
P2O5	42.12	40.04	39.18	39.29		37.72	37.87		42.12	41.59		37.77		41.41		41.35
SiO2	0.038	0.13	0.1	0.18		0.83	1.41		0.11	0.13		0.13		-		0.11
Na2O	0.125	0.01	0.02	0.11		0.486	0.36		BD	BD		-		-		0.01
CaO	53.88	55.19	55.7	55.32		54.63	53.33		54.21	54.42		54.44		53.47		54.99
SrO	0.433	0.98	0.88	0.79		0.29	0.3		0.03	0.07		1.29		1.18		0.04
La2O3	0.58	0.21	0.29	0.37		0.47	0.084		BD	BD		0.1		-		-
Ce2O3	0.62	0.58	0.49	0.67		0.66	0.256		BD	BD		0.22		-		-
F	2.08	2.8	3.23	3.13		1.08	1.02		2.47	2.45		3.87		3.21		2.37
Cl	0.028	0.039	0.01	0.04		0.01	0.01		1.17	1.33		-		-		1.4
Total	99.904	99.979	99.9	99.9		96.176	94.64		100.11	99.99		97.82		99.27		100.27
P	5.89	5.664	5.569	5.585		5.583	5.627		5.851	5.805		5.492		5.816		5.772
Si	0.006	0.022	0.017	0.03		0.145	0.247		0.018	0.021		0.022		-		0.018
Na	0.04	0.003	0.007	0.036		0.165	0.123		0	0		-		-		0.003
Ca	9.535	9.881	10.019	9.951		10.232	10.028		9.53	9.613		10.017		9.504		9.714
Sr	0.052	0.095	0.086	0.077		0.029	0.031		0.003	0.007		0.128		0.114		0.004
La	0.035	0.013	0.018	0.023		0.03	0.005		0	0		0.006		-		-
Ce	0.037	0.035	0.03	0.041		0.042	0.016		0	0		0.014		-		-
F	1.086	1.48	1.715	1.662		0.597	0.566		1.282	1.277		2.102		1.684		1.236
Cl	0.008	0.011	0.012	0.011		0.003	0.003		0.325	0.372		-		-		0.391
F/Cl (at)	139	134	147	146		202	190		4	3		-		-		3
Data source: a (Xu et al., 2010); b (Vrublevskii et al., 2018); c (Le Bas et al., 2002).

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Pyroxene analyzed in both the meta-carbonatites and calc-silicate marbles are diopside (Table 3). The diopside in the meta-carbonatites contains higher Na₂O (0.96 wt.%–1.60 wt.%) and lower in Al₂O₃ (0.12 wt.%–1.15 wt.%) contents when compared to those in the calc-silicate marbles (Na₂O = 0.22 wt.%–0.56 wt.% and Al₂O₃ = 2.05 wt.%–6.34 wt.%). In addition, the pyroxenes in the calc-silicate marble have Mg^# of 90–91, which is significantly higher than those in the meta-carbonatites Mg^# of 77–88.

Table  3.  Representative major element (wt.%) analysis of diopside in Jambil carbonatites and calc-silicate marbles

	Carbonatites							Calc-silicate marble (Meragai)				Delbek Carbonatite (Turkestan)b		Borra Marble (India)c
	JS-46				JS-52			JS-49
	1	2	3		4	5		9	10	12		Av (2)		Av (9)
SiO2	53.97	52.56	53.29		54.62	54.18		50.35	49.75	52.13		52.69		49.73
TiO2	BD	0.51	0.42		0.17	0.29		0.78	0.87	0.31		-		0.77
Al2O3	0.12	1.14	1.15		0.26	0.65		4.26	6.34	2.05		-		5.17
Fe2O3	1.42	2.81	1.42		2.89	1.71		BD	BD	BD		-		-
FeO	2.31	4.28	3.17		2.7	3.11		2.34	2.55	2.44		1.31		2.54
MnO	0.2	0.34	0.33		0.16	0.14		0.16	0.15	0.39		0.24		0.17
MgO	15.28	13.54	15.11		14.91	15.06		14.7	14.31	15.81		17.4		14.3
CaO	24.11	23.15	23.7		22.92	23.13		25.31	25.54	25.69		25.26		25.42
Na2O	1.28	1.19	0.96		1.6	1.2		0.29	0.56	0.22		-		0.41
Total	98.69	99.52	99.55		100.23	99.47		98.19	100.07	99.04		96.9		98.51
Si	2.003	1.958	1.966		1.999	1.994		1.871	1.81	1.919		1.97		1.842
Ti	0	0.014	0.012		0.005	0.008		0.022	0.024	0.009		-		0.021
Al	0.005	0.05	0.05		0.011	0.028		0.187	0.272	0.089		-		0.226
Fe+3	0.04	0.079	0.039		0.08	0.047		0.049	0	0.072		-		-
Fe+2	0.072	0.133	0.098		0.083	0.096		0.024	0.078	0.003		0.041		0.079
Mn	0.006	0.011	0.01		0.005	0.004		0.005	0.005	0.012		0.008		0.005
Mg	0.845	0.752	0.831		0.813	0.826		0.814	0.776	0.868		0.97		0.789
Ca	0.958	0.924	0.937		0.899	0.912		1.008	0.996	1.013		1.012		1.009
Na	0.092	0.086	0.069		0.114	0.086		0.021	0.04	0.016		-		0.029
Mg#	88	77	85		83	85		91	90	91		95		90
Data source: b (Vrublevskii et al., 2018); c (Le Bas et al., 2002).

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Feldspars in the meta-carbonatites are similar to albite (Ab = 95–98) and microcline (Or = 96) compared to the anorthitic (An = 91–95) composition in the calc-silicate marble (Table 4). The alkali feldspar of the Jambil meta-carbonatites contain an order of magnitude more BaO (0.6 wt.%–1.1 wt.%) than those in the calc-silicate marble (0.02 wt.%–0.06 wt.%). High BaO content is considered as a typical characteristic of carbonatites and alkaline rocks (Kapustin, 1980).

Table  4.  Representative major element (wt.%) analysis of feldspar in Jambil carbonatites and calc-silicate marbles

	Carbonatites						Calc-silicate marble (Meragai)			Delbek Carbonatite (Turkestan)b		Borra Marble (India)c
	JS-46		JS-52				JS-49
	2		6	7	8		9	11		Av (9)	Av (7)
SiO2	61.31		59.19	63.97	64.43		44.71	45.57		63.28	68.18	42.93
TiO2	BD		BD	BD	BD		0.26	0.25		-	-	-
Al2O3	20.31		20.09	17.88	17.13		34.26	33.65		17.7	18.82	35.08
FeO	0.1		0.39	0.23	0.54		0.24	0.39		0.35	0.32	0.06
MnO	BD		BD	BD	BD		0.13	0.13		-	-	-
MgO	BD		BD	BD	BD		0.06	0.08		-	-	-
CaO	0.91		0.04	0.21	0.19		19.51	17.7		-	0.23	20.55
SrO	0.21		0.32	0.36	0.41		BD	BD		-	-	0.12
BaO	1.1		0.6	0.72	0.65		0.02	0.06		0.76	-	BD
Na2O	10.11		9.7	0.33	0.36		0.29	0.89		0.23	11.35	0.44
K2O	0.09		0.23	16.23	16.23		0.51	0.15		16.66	0.24	-
Total	94.14		90.56	99.93	99.94		99.99	98.87		98.98	99.14	99.18
An	5		0	1	1		95	91		0	1	96
Ab	95		98	3	3		3	8		2	98	4
Or	1		2	96	96		3	1		98	1	0
Data source: b (Vrublevskii et al., 2018); c (Le Bas et al., 2002).

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3.2 Whole Rock Geochemistry

The whole rock chemical data: from the meta-carbonatites and calc-silicate marbles sampled are presented in Table S1. Compositionally, the CaO-MgO-FeO^t + MnO triangular classification diagram of Woolley and Kempe (1989) shows that the Jambil meta-carbonatites are calcio-carbonatites (Fig. 5a). Most of the meta-carbonatites are characteristically higher in MnO and P₂O₅ compared to the calc-silicate marble (Table S1). However, other major oxides lack appreciable differences to discriminate between the carbonatite and marble.

Figure  5.  The Jambil meta-carbonatites plot within the calcio-carbonatite field in the CaO-MgO-(FeOt + MnO) classification diagram (a) of Woolley and Kempe (1989), and (b) a Sr + Ba vs REEs + Y diagram (after Samoilov, 1991). Calc-silicate marble specimens plot in the field of sedimentary and metamorphic rocks origin in the same trace element plot.

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In contrast to the major oxides, trace and REE demonstrate significant differences between the Jambil meta-carbonatites and calc-marbles (Figs. 5b and 6). On the Samoilov (1991) Sr + Ba vs REE + Y plot, the Jambil meta-carbonatites plot within the carbonatite field and the calc-silicate marble occupy the sedimentary and metamorphic rock fields (Fig. 5b). The Jambil meta-carbonatites have a higher trace element concentration (with the exception of Rb and Ba) than the calc-silicate marble, which shows similarities to the calc-silicate Borra marble, India (Le Bas et al., 2002) (Fig. 6a). The Jambil meta-carbonatites also have higher overall REE concentrations than the marble (Fig. 6b), similar to average calico-carbonatites worldwide (Woolley and Kempe, 1989).

Figure  6.  (a) Primitive mantle normalized trace elements and (b) chondrite normalized REEs diagrams of the Jambil meta-carbonatites and calc-silicate marbles, compared to the average calcio-carbonatite and Borra marble (India) from Woolley and Kempe (1989) and Le Bas et al. (2002), respectively. Normalization values are from McDonough and Sun (1995).

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The meta-carbonatites are depleted in Rb, Nb, Ta, Ti, Zr and Hf, and relatively enriched in Ba, Th, LREE and Sr, which is common in other carbonatites (Hou et al., 2006; Woolley and Kempe, 1989; Nelson et al., 1988) (Fig. 6a). The chondrite-normalized REE patterns of the meta-carbonatites show enrichment of LREE over HREE (Fig. 6b) with a high (La/Yb)_CN = 35–106 (Table S1).

3.3 Carbon and Oxygen Isotope Composition

The carbon and oxygen isotope compositions of the Jambil meta-carbonatites vary from -3.8‰ to -4.5‰ δC_PDB¹³ and from 9.3 to 11.7 δO_SMOW¹⁸, respectively (Fig. 7). The carbon and oxygen isotopic values of four carbonatite samples plot inside the generally accepted field of primary, unaltered igneous carbonatites, identified by Taylor et al. (1967). Whereas, C-O isotope values of four other carbonatite samples plot outside the field of primary, unaltered igneous carbonatites, as well as out of the typical mantle source range (Ray et al., 1999). The carbon and oxygen isotopic values of calc-silicate marble plot in the field of Paleoproterozoic sedimentary carbonate rocks (Veizer et al., 1992). The trend shown by the Jambil meta-carbonatites in Fig. 7, are comparable with the trends of carbonatites produced by either high temperature fractionation, contamination or carbonatites derived from sedimentary carbonates protolith (Demény et al., 1998; Deines, 1989).

Figure  7.  Carbon and oxygen isotopic composition of the Jambil meta-carbonatites. Fields of primary igneous carbonatite, typical mantle and Proterozoic sedimentary carbonates are after Taylor et al. (1967), Ray et al. (1999) and Veizer et al. (1992), respectively. Directions of arrows represent main processes responsible for changes in the isotopic compositions (after Deines, 1989).

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3.4 U-Th/Pb Geochronology

Titanite in the meta-carbonatites (specimen J52) appears euhedral, golden to dark brown, unzoned in back-scattered electron images and over 100 µm long (Figs. 3a, 3c). Fifteen titanite grains were selected for in-situ U-Pb dating. The LA-ICP-MS U/Pb data from fifty-one spots on titanite has a large spread in Terra-Wasserburg space. A robust old population with an intercept age of 437 ± 3 Ma (MSWD = 2.1) can be extracted from the data (Fig. 8a; Table S2).

Figure  8.  (a) Tera-Wasserburg concordia diagram of titanite analyses. An older population of datapoints, used in the age regression, is shown in color whereas the grey dashed ellipses are not included. (b) U-Th-Pb plot of monazite data from the carbonatite with an inset weighted mean ²³²Th/²⁰⁸Pb age plot.

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Most of the monazite grains in the meta-carbonatites (specimen J48) appear subhedural and range in size from 150–330 µm (Figs. 3b, 3d). Forty-six spot analyses from seven monazite grains yield reversely discordant data in U-Th-Pb space (Fig. 8b). This spatial pattern likely reflects excess ²⁰⁶Pb from the decay of unsupported ²³⁰Th (Schärer, 1984). Because of the reverse discordance we prefer to use the ²⁰⁸Pb/²³²Th ages which range from 40.3 to 27.6 Ma with a weighted mean (excluding the youngest analysis) of 38 ± 0.2 (MSWD = 5.8, n = 42/43) (Fig. 8b; Table S3). The high MSWD on the weighted mean indicates that the dates do not comprise a single population and may indicate progressive crystallization.

4. DISCUSSION

4.1 Carbonatite or Calc-Silicate Marble?

Meta-carbonatites can be challenging to distinguish from common marbles in the field, particularly when both are metamorphosed during amphibolite and higher grades of metamorphism. During metamorphism, both rock types can develop a mosaic texture of interlocking calcite grains with well-developed triple junction boundaries. Moreover, the mineral assemblage of diopside-magnetite-apatite with calcite is common in both igneous carbonatites and calc-silicate marble of sedimentary protolith. In most cases, the mineral assemblages cannot be used to differentiate the two rock types, but it is possible to discriminate between them based on characteristics such as higher SrO > 0.15 wt.% in calcite and low Al content of other constituent minerals in carbonatites (Yang and Le Bas, 2004). Aluminous minerals like anorthite, Al-rich diopside, scapolite or spinel are common in marbles and are rare in carbonatites. Higher Na₂O content of pyroxene are also typical of igneous paragenesis. The high Na₂O in pyroxene of the Jambil meta-carbonatites (0.96% to 1.60%) is closely similar to the pyroxene with > 1% Na₂O reported from other carbonatite complexes (Chakhmouradian et al., 2008; Moecher et al., 1997; Viladkar and Subramanian, 1995; Natarajana et al., 1994; Kapustin, 1980). Iron rich pyroxene with < 1% Na₂O and Al₂O₃ is a diagnostic signature to differentiate pyroxene of skarn from the pyroxene of calc-silicate marbles and meta-carbonatites (Nakano et al., 1994). In addition, the Mg^# also discriminate calc-silicate marble from meta-carbonatites (Le Bas et al., 2002). Pyroxene in marble has Mg^# > 90, whereas, in most carbonatites and alkaline igneous rocks, the Mg^# is < 90 (Lloyd et al., 1999; Deer et al., 1997). Consequently, low Al₂O₃, high Na₂O contents and a lower Mg^# (i.e., < 90) in the ferromagnesian minerals of meta-carbonatites can be used to discriminate between the two rock types. Moreover, the F/Cl ratio in apatite (134–202) of the Jambil meta-carbonatites is consistent with higher F/Cl ratio (i.e., > 59) found in typical carbonatitic apatite (Smith et al., 1981). Apatite in marble have a higher Cl/F ratio and are slightly poor in Sr, Na, Si and REE compared to apatite in carbonatites and even in other igneous silicate rocks (Barker, 1996).

The trace, REE and C-O isotope composition of meta-carbonatites are unique and provide the most convincing evidence of their igneous origin. Meta-carbonatites are characteristically higher in all trace and REE analyzed compared to calc-silicate marbles and are similar to average calcio-carbonatites in the world (Chakhmouradian et al., 2008; Woolley and Kempe, 1989) (Table S1; Fig. 6). More than 0.5 wt.% SrO in carbonatite rocks is also a good indicator to distinguish meta-carbonatites from calc-silicate marble (Le Bas et al., 2002).

4.2 Petrogenesis of Jambil Meta-Carbonatites

Carbonatites are usually associated with alkaline igneous rocks, and several processes proposed for their genesis include liquids immiscibility (Lee and Wyllie, 1998, 1997a, b; Hamilton et al., 1989; Kjarsgaard and Hamilton, 1989) and extensive crystal fractionation of CO₂-rich silicate magma (Verhulst et al., 2000; Veksler et al., 1998; Lee and Wyllie, 1994). However, the absence of alkaline rocks in vicinity of the Jambil meta-carbonatites puts into question the role of liquid immiscibility and fractional crystallization during its formation. Alternatively, it is possible that the parental melt for Jambil meta-carbonatites is primary and generated directly from small degree partial melting of carbonate-bearing source such as carbonated mantle or carbonated eclogite. Low degree partial melting of carbonated mantle peridotite can generate magnesion, essentially dolomitic, melt with a SiO₂ content less than 5%–10% (Wyllie and Lee, 1998; Wyllie, 1989; Wallace and Green, 1988; Wyllie and Huang, 1975). This dolomitic melt could produce calico-carbonatites similar to Jambil by decarbonation reaction in mantle lherzolite and harzburgite zones at lower pressure, while ascending through the mantle from site of generation (Dalton and Wood, 1993). The resultant calcio-carbonatites melt would contain up to 18% silicate contents (Wyllie and Lee, 1998). However, the relative lower SiO₂ contents of the Jambil meta-carbonatites contradict its genesis by such processes.

Carbonate and volatile-rich oceanic crust subducted at 45 km or greater can generate carbonated eclogite with garnet, pyroxene and accessory titanite as the major mineral phases (Manthilake et al., 2008; Yaxley and Brey, 2004; Hammouda, 2003). Previous studies on carbonatite complexes and experimental evidence demonstrates that a low degree partial melting (< ≈1%) of garnet-rich carbonated eclogites can generate calcio-carbonatites (Dasgupta et al., 2005; Dasgupta et al., 2004; Yaxley and Brey, 2004; Hammouda, 2003; Hoernle et al., 2002; Yaxley and Green, 1994; Nelson et al., 1988). Under such conditions, enrichment of LREE over HREE is a primary magmatic feature of carbonatites (Xu et al., 2010; Ying et al., 2004). The depletion of HREE observed in the Jambil meta-carbonatites is consistent with garnet-bearing eclogitic source, where garnet retained HREE during partial melting and fractionation/enrichment of LREE occurred in the resultant melt (Xu et al., 2007; Nelson et al., 1988). A carbonated eclogite source for the Jambil meta-carbonatites is also consistent with the depleted concentrations of Nb, Ta, Ti, Zr and Hf, and relative enrichment of Th, which require saturation of mineral phases such as titanite and rutile in the source. These minerals are common in eclogite but not peridotite (Xu et al., 2015; Hoernle et al., 2002; Clague and Frey, 1982). Moreover, the extreme depletion of Cr and Ni in the Jambil meta-carbonatites also favors an eclogite source, whereas high Cr and Ni concentrations are distinctive of melts derived from peridotite source (Xu et al., 2014; Manthilake et al., 2008). Subducting marine sedimentary carbonates overlying the magmatic oceanic crust as well as secondary calcium carbonate occurring as veins, vesicles, filling void spaces, lavas flows and pillows, may be an effective mechanism of supplying carbonate to the mantle (Trull et al., 1993; Staudigel et al., 1989). Such subducting marine carbonates can survive to temperatures and pressures of > 900 ℃ and > 35 kbar, respectively, lingering as a refractory phase in rutile eclogite (Yaxley and Green, 1994). The existence of aragonite associated with high pressure minerals in eclogite facies rocks provides a natural evidence of high P-T stability of carbonates (Becker and Altherr, 1992; Smith, 1988). Consequently, such recycled carbonates through the mantle with the oceanic crust after being converted to eclogite may be sources for meta-carbonatites (Hoernle et al., 2002).

The carbon and oxygen isotopic compositions of four samples of the Jambil meta-carbonatites cluster outside at the upper right corner of the primary, unaltered igneous carbonatite field (Taylor et al., 1967). However, four samples with isotopic ratios plotting inside the primary, unaltered igneous carbonatites field (Fig. 7). Deines (1989) argues that the carbon isotopic values of primary, unaltered igneous carbonatites represent a mantle source. The C and O isotopic compositions of Jambil meta-carbonatites imply that the body is generated from melting of an isotopically heavier carbon source. Such enrichment of a mantle source with heavier carbon is possible by assimilation of inorganic carbon (δ¹³C = 0) with primordial carbon (δ¹³C = -6‰) during the subduction process (Ray et al., 1999). Similarly, eclogites have δ¹³C_PDB of -34‰ to +4‰ with an average value plotting in the field of primary igneous carbonatites; therefore, these isotopes are consistent with the Jambil meta-carbonatites being generated by a small degree of partial melting of an eclogitic source (Hoernle et al., 2002; Pearson et al., 1994). Thus, crustal carbon recycled to the mantle via subduction of the Proto-Tethyan oceanic crust is the most likely source of the heavy-C enrichment in the Jambil meta-carbonatites. However, the stable isotope data are not conclusive in this regard, given that the Jambil meta-carbonatites have underwent metamorphism/metasomatism, therefore, the C and O isotopic compositions might not reflect the source. The variable carbon and oxygen isotopic compositions may be result of Rayleigh crystal fractionation giving rise to fluid exsolution from carbonatite in high temperature and or interaction of carbonatite with meteoric water at relatively low temperature (Ray and Ramesh, 2000; Santos and Clayton, 1995). Rayleigh-type fractionation more likely played the dominant role in the generation of variable C and O isotopic compositions recorded in the Jambil meta-carbonatites, with the assumption that some calcites with higher C and O isotopic values are hydrothermal and precipitated by the carbonatite-exsolved fluids (Ying et al., 2020).

4.3 Emplacement and Metamorphic Timing

There is no previous record of U-Pb geochronology from the Jambil meta-carbonatites. The only published data include a fission track apatite age of 29.3 ± 1.2 Ma, which is interpreted as the crystallization age of meta-carbonatites (Khattak et al., 2004). The new U-Pb titanite data presented herein provide the first estimate of crystallization of the carbonatite at ca. 437 ± 3 Ma. The monazite ages from the carbonatite yielded a weighted mean of ~38 Ma with a large MSWD. We interpret that age to reflect protracted growth during metamorphism of the meta-carbonatites. The age overlaps with monazite ages recently reported by Larson et al. (2019), from the same area, which range between 39 and 28 Ma. The previously published apatite fission track date is reinterpreted to reflect the early stages post-peak metamorphic cooling.

4.4 Tectonic Implication and for Proto-Tethyan Subduction

Prior to the Cenozoic Himalayan orogeny, the Himalayan region of Northwest Pakistan was part of the northern Gondwana supercontinent during Early Paleozoic time (Sajid et al., 2018; Cawood et al., 2007). Several other micro-continental blocks including Simao-Indochina, Lhasa, Sibumasu, North and South Qiangtang, Qilian, North Qinling and South China were believed to have amalgamated to the northern Gondwana margin, leading to the culmination of eastern Gondwana during the Paleozoic (Wang et al., 2020, 2019; Li et al., 2018; Metcalfe, 2013; Cawood et al., 2007). The amalgamation and collision of these blocks occurred due to subduction of the Proto-Tethyan beneath the northern Gondwana margin (Cawood et al., 2007) (Fig. 9a). The Andean-type subduction margin resulted in extensive magmatism along the northern Gondwana terrains during the Early Paleozoic, including those now exposed in Turkey (Şahin et al., 2014; Gürsu, 2008), Iran (Moghadam et al., 2015), Pakistan (Qasim et al., 2018, 2015; Sajid et al., 2018), and in Southern Tibet (Hu et al., 2015, 2013; Zhang et al., 2015; Wang Y X et al., 2013; Wang X X et al., 2012). In addition, the Cambro-Ordovican Mandi and Tso-Morari pluton from Northwest India (Miller et al., 2001; Girard and Bussy, 1999; respectively), Utla and Mansehra granites, and Kafiristan pluton from North Pakistan (Sajid et al., 2018; and Faisal et al., 2016; respectively) and granitic plutons from the Baoshan and Shan-Thai Block from Southwest China (Dong et al., 2013; Wang et al., 2013, respectively) mark Early Paleozoic magmatism in an arc-back-arc setting due to Proto-Tethys subduction and slab roll-back activities along the northern margin of Gondwana (Fig. 9b). The arc-back-arc magmatism lasted through the Silurian and possibly into the Early Devonian as evidenced by the Chenjiahe Group and Hongtubao Formation volcanics in North Qilian, and bimodal Dawazi and Dazhonghe volcanism in SW Yunnan, China (Liu et al., 2019; Li et al., 2018). Geochemical data from Ordovician calc-alkaline peraluminous granites (Mansehra and Utla granites), in the vicinity of study area have been explained to record subduction of the Proto-Tethyan oceanic crust to the south beneath the northern margin of Gondwana and existence of the back-arc setting (Sajid et al., 2018; Cawood et al., 2007) (Fig. 9b). Moreover, the Early Paleozoic quartzites, argillites and carbonates in the Peshawar Basin and Abbottabad area, North Pakistan are consistent with sediment accumulation in a back-arc basin environment during that time (Palin et al., 2018; Qasim et al., 2018; Pogue et al., 1992).

Figure  9.  Schematic tectonic diagram showing the evolution model of the northern margin of Gondwana and emplacement of the Jambil carbonatites during the Paleozoic time (a)–(c) (modified from Sajid et al., 2018), and metamorphism of the carbonatites due to the Himalayan collision (d).

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An unconformity between the Ordovician and Cambrian strata and Early Paleozoic metamorphism (482.4 ± 7.9 and 464.5 ± 4.0 Ma, Palin et al., 2018) in northern Pakistan also support the existence of an Early Paleozoic tectonothermal event. This Early Paleozoic magmatism reflects subduction and recycling of the Proto-Tethyan oceanic crust and marine sedimentary carbonates into the mantle (Trull et al., 1993; Staudigel et al., 1989). A low degree of partial melting of recycled carbonates and oceanic crust (after being converted to carbonated eclogite), during the Cambro-Ordovician, is interpreted to have generated the carbonatite melt emplaced as the Jambil carbonatite likely in the same back-arc extensional setting during the Early Silurian (437 ± 3 Ma) (Fig. 9c). The emplacement of the Jambil meta-carbonatites mark a transition from a compressional tectonic regime, brought about by subduction of the Proto-Tethys oceanic subduction and subsequent collision of micro-continental blocks along the northern margin of Gondwana, to post-orogenic extension in the waning stages of the pre-Himalayan Ordovician orogeny.

During the Cenozoic, the Jambil meta-carbonatites and environ were subjected to regional deformation and metamorphism as a part of the Himalayan Orogen (Fig. 9d). The in-situ monazite population ca. 38 Ma records metamorphism of the meta-carbonatites at the time.

5. CONCLUSIONS

In this study, we present geochemical data to differentiate meta-carbonatites from calc-silicate marble, specifically focusing on the Jambil meta-carbonatites in northern Pakistan. The data presented outline the petrogenesis of the Jambil meta-carbonatites as the product of primary carbonatitic melt generated by low degree partial melting of carbonated eclogite during the Early Silurian, probably in back-arc extensional setting. During Cenozoic, the Jambil carbonatites were metamorphosed to meta-carbonatites as part of the ongoing India-Asia collision. Although, the Paleozoic events are obscured by the younger Himalayan tectonics, the petrogenetic history of the Jambil meta-carbonatites, together with existing geological observation provide insights into the tectonic evolution of the northern Indian continental margin during the Paleozoic time.