Hidden Beneath Bangladesh’s Sediments, a 27-Metre Slice of Ancient Crust Comes to Light

Deep beneath the rice paddies and river deltas of northwestern Bangladesh, drill hole GDH-62 has quietly delivered one of the region’s most detailed portraits of its buried Precambrian foundations. In a new open-access study published in Discover Geoscience, researchers led by Hasibul Zahan of Bangladesh Water Development Board and Saitama University, together with colleagues at Curtin University, Jahangirnagar University and the University of Otago, describe a continuous 27.5-metre core of crystalline basement rock recovered from depths of 496.8 to 524.3 metres in Dighipara, Dinajpur District. The core, recovered with better than 95 percent recovery, reveals a dominantly felsic basement of granodiorite pierced near its base by a roughly 7.3-metre-thick appinitic dyke, a combination that adds a striking new piece to the puzzle of how this corner of the Indian Shield was assembled.

The setting alone makes the work compelling. Bangladesh sits atop the Bengal Basin, one of the world’s great sedimentary repositories, where strata eroded from the rising Himalaya and carried by the Ganges, Brahmaputra and Meghna rivers have buried the ancient crust under hundreds to thousands of metres of Late Permian to Quaternary sediment. In the northwest, however, the Precambrian basement rises unusually close to the surface. The shallowest known occurrence lies at just 128 metres depth at Maddhapara, where diorite-tonalite-granodiorite rocks have been dated at 1.64 to 1.73 billion years using U-Pb SHRIMP analysis of zircon. That age places the Dinajpur Block firmly within the Paleoproterozoic, an era when the planet’s landmasses were assembling into the supercontinent Columbia, also known as Nuna.

Until now, drill hole GDH-62 had never been examined petrographically. The team collected nine core samples from the Geological Survey of Bangladesh repository and prepared twenty-seven thin sections, two or three per sample to capture mineralogical variation along and across the core axis. Under a standard polarizing microscope, they identified minerals and counted them along twenty-five to thirty-five grid-based traverses per section, estimating modal compositions visually with an uncertainty of about plus or minus five percent for major minerals. The approach is deliberately classical, but it is exactly the kind of careful, quantitative petrography that turns a stack of grey core boxes into a readable geological history.

The results split cleanly into two rock families. The felsic rocks, which occupy about 20.2 metres of the core or roughly 73 percent of the studied interval, are granodiorite: massive, grey to greenish grey, leucocratic to mesocratic, and fine to medium grained. Their modal composition ranges from 33.4 to 49.2 percent plagioclase, 11.8 to 27.0 percent quartz, 6.6 to 27.2 percent biotite and 6.7 to 11.7 percent K-feldspar, with secondary chlorite and epidote and accessory muscovite, zircon, hematite and opaque minerals. Plotted on the standard Quartz-Alkali Feldspar-Plagioclase discrimination diagram, every felsic sample falls within or adjacent to the granodiorite field, some near the granodiorite-tonalite boundary. The textures are hypidiomorphic and inequigranular, with fine- to medium-grained interlocking fabrics that speak of slow crystallization in a deep plutonic environment.

The microscopic detail is where the rocks begin to tell their deformation story. Quartz grains show wavy, undulose extinction, a classic signature of strain accumulated in the crystal lattice, and locally the quartz has recrystallized into finer equant aggregates. Biotite occurs as blade-like, euhedral to subhedral crystals with light brown to yellowish brown pleochroism, and many grains are twisted, bent and partially foliated. Aligned biotite and chlorite along recrystallized quartz grain boundaries define a weak foliation whose intensity, notably, decreases with depth in the section, hinting at a gradient in deformation intensity that the authors caution cannot be quantified from petrology alone. K-feldspar displays textbook Carlsbad twinning and hosts inclusions of acicular muscovite, biotite and rare epidote, the latter possibly pointing to hydrothermal alteration. Plagioclase shows weak to moderate sericitization, and much of the biotite is altered to chlorite, evidence that fluids, possibly of hydrothermal origin, percolated through these rocks long after they crystallized.

Cutting through the granodiorite near the base of the hole is the study’s most eye-catching discovery: an appinite dyke, massive, greenish grey to very dark green, and dominated by hornblende at 67 to 73 percent of its mode, accompanied by biotite, plagioclase, chlorite, hematite and accessory opaque minerals, with calcite forming micro-veins and veinlets. Appinites are an unusual and petrogenetically significant class of rock, typically hornblende-rich mafic intrusions associated with hydrous, mantle-derived magmas in convergent settings. In thin section the appinite shows hypidiomorphic, inequigranular and locally poikilitic textures, with hornblende enclosing plagioclase crystals. Hornblende exhibits the characteristic amphibole cleavage angles of 56 and 124 degrees with extinction angles between 12 and 34 degrees, while reddish brown biotite suggests a titanium-rich composition that the authors flag as potentially significant for petrogenesis. A tentative identification of orthopyroxene, based on pleochroism and parallel extinction, remains unconfirmed without optical or chemical constraint on its 2V angle.

The contact between the dyke and its host is sharp and non-gradational, with no chilled margin, and the authors interpret it as evidence that the appinite was emplaced after the felsic host had consolidated, most likely as a hypabyssal dyke within the main acid to intermediate association. From granodiorite to appinite, quartz and plagioclase decrease while biotite increases, tracking the shift from felsic to mafic chemistry. What petrography cannot constrain, the researchers emphasize, is the emplacement depth, a limitation they state plainly rather than paper over. That honesty matters in a field where over-interpretation of thin-section evidence is a chronic hazard.

Perhaps the most technically rich part of the study is its analysis of vein microstructures, which function as a kind of strain gauge for events long after the magmas cooled. In the granodiorite, lensoidal quartz veins with smooth vein-wall interfaces and lensoid wall-rock fragments point to a fracture-controlled vein system formed by syntaxial growth, in which crystals grow from both walls toward the vein centre. The small angle between the growth phase and the crack wall suggests a low growth rate, and the absence of fluid inclusions indicates that the vein crystals were deformed under non-hydrostatic stress during or after formation. In the appinite, by contrast, monomineralic calcite veins show slightly curved growth and selvages at their margins, hallmarks of antitaxial growth in which crystals expand outward from the vein centre, recording progressive, stress-related deformation over time. Minor calcite veins oriented obliquely and sub-parallel to the major vein hint at changing stress directions and possible fracture reactivation across the life of the intrusive complex.

The comparison the authors draw with the West African Craton gives the work its global resonance. Paleoproterozoic granitoids of the Bas Draa inlier in Morocco’s Anti-Atlas, dated around 2.1 billion years, share calc-alkaline, subduction-related characteristics with the Dinajpur Block, and both regions feature sodic TTG-style suites derived from partial melting of hydrated mafic sources. Mafic and appinitic dykes in GDH-62 resemble the lamprophyric dyke swarms of the West African Craton, though the Bangladeshi examples point more strongly to metasomatic and mantle-wedge influences than to plume-related processes. The similarities support the interpretation of GDH-62 as a fragment of Paleoproterozoic arc crust, a piece of a convergent-to-collisional margin that helped stitch together Columbia. For a country whose geology is usually described in terms of deltas, floods and gas fields, the message is striking: beneath the youngest sediments lies a two-billion-year-old arc, complete with dykes, veins and deformation histories, waiting in core boxes for anyone patient enough to grind the thin sections.

Subject of Research: Petrographic characterization of Precambrian crystalline basement rocks in drill hole GDH-62, Dighipara, northwest Bangladesh

Article Title: Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh

Article References: Zahan, H., Ameen, S. M. M., Hossain, M. S., & Hossain, M. S. (2026). Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh. Discover Geoscience, 4(1), Article 332. https://doi.org/10.1007/s44288-026-00703-2

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00703-2

Keywords: petrography, crystalline basement, granodiorite, appinite, Bangladesh, Dinajpur Block, Bengal Basin, Precambrian, vein microstructures, Columbia supercontinent, drill core, hornblende