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Papers by Saumitra Misra

Journal of Earth System Science

DHALA IMPACT STRUCTURE, INDIA. Md. Arif, S. K. Patil, J. K. Pati and S. Misra, Birbal Sahni Insti... more DHALA IMPACT STRUCTURE, INDIA. Md. Arif, S. K. Patil, J. K. Pati and S. Misra, Birbal Sahni Institute of Palaeosciences, Lucknow-226007, India (arif@bsip.res.in), Dr. K. S. Krishnan Geomagnetic Research Laboratory, IIG, Allahabad-221505, India, Department of Earth & Planetary Sciences, University of Allahabad, Allahabad-211002, India, SAEES, University of KwaZulu-Natal, Durban-4000, South Africa (misras@ukzn.ac.za).

Journal of the Geological Society of India

ABSTRACT

Meteoritics & planetary science

The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps b... more The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (~65 Ma) at ~570±47 ka ago by an oblique impact of a possible chondritic asteroid that stuck the pre-impact target from the east at an angle of ~30-45o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east-west plane of impact (Misra et al., 2010). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g. low-field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high coercivity and high temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K3 (minimum) AMS axes of shocked basalts from around the cr...

Journal of the Geological Society of India, 2009

... We are indebted to Kailas Sekhar Banerjee, Indian Institute of Technology, Kharagpur, for his... more ... We are indebted to Kailas Sekhar Banerjee, Indian Institute of Technology, Kharagpur, for his excellent help during sampling for this work; to B. Das, P. Guha and BPMirddya, XRD laboratory, CRF, IIT Kharagpur, for their expert guidance during XRD analyses; and to the Head ...

Geological Magazine, 2010

Gabbro samples (c. < 0.4 Ma old) dredged from close to the ‘Vityaz Megamullion’ on the slow-sp... more Gabbro samples (c. < 0.4 Ma old) dredged from close to the ‘Vityaz Megamullion’ on the slow-spreading Northern Central Indian Ridge (NCIR, 18–22 mm yr−1) include mostly olivine gabbro and Fe–Ti oxide gabbro. The cumulate olivine gabbro shows ophitic to subophitic texture with early formed plagioclase crystals in mutual contact with each other, and a narrow range of compositions of olivine (Fo80–81), clinopyroxene (magnesium number: 85–87) and plagioclase (An67–70). This olivine gabbro could be geochemically cogenetic with the evolved oxide gabbro. These gabbro samples are geochemically distinct from the CIR gabbro occurring along the Vema, Argo and Marie Celeste transform faults and can further be discriminated from the associated NCIR basalts by their clinopyroxene (augite in gabbro, and diopsidic in basalts) and olivine (gabbro: Fo80–81, basalts: Fo82–88) compositions. Our major oxide, trace element and REE geochemistry analyses suggest that the gabbro and the NCIR basalts are ...

Nanoparticles formed within the ejecta plume are thought to play an important role in the catastr... more Nanoparticles formed within the ejecta plume are thought to play an important role in the catastrophic events due to meteorite impacts. However, the mode of origin of these nanoparticles in the ejecta fallout is still not clearly known due to its poor preservation potential and alteration. Conclusions The presence of single domain crystalline nanoparticles, i.e. non-biogenic titanomagnetite with distinct hexagonal and octahedral crystal habit, in the ejecta fallout supports its formation due to vapor-solid condensation within the impact ejecta plume and the prevalence of the metallic phases (i.e. Fe and Ti) in the ejecta plume probably facilitated the nano-phase crystallization. Objectives To understand the development of the nano-phase in the ejecta fallout, isolated magnetic particles from the ejecta blanket of the Lonar Crater in India Methods To understand the development of the nano-phase in the ejecta fallout, isolated magnetic particles from the ejecta blanket of the Lonar Crater in India were analyzed using a transmission electron microscope (TEM) and EELS. a high sensitivity scanning electron microscope (SEM) equipped with energy dispersive spectrum analyzer (EDS). Methods Isolated magnetic particles from the ejecta blanket of the Lonar Crater in India were obtained through manual separation of magnetic particles, sieving, washing the less than 62 um size fraction with 7N HCL, followed by sonication and settling in deionized water. The isolated magnetic particles were analyzed using a transmission electron microscope (TEM) and equipped with electron energy loss spectroscope (EELS), to determine the composition of the nanoparticles. TEM analysis was carried out in a Libra 120 Zeiss system operating at 120 kV with a point to point resolution of 0.34 nm. The images were taken using bright field imaging and selective area electron diffraction was performed in an area of 100 nm with high population of nanoparticles.

Journal of the Geological Society of India

Meteoritics & Planetary Science

The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angu... more The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejec...

Tectonics and Structural Geology: Indian Context, 2018

The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-... more The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-lying Vindhyan Supergroup of sedimentary rocks of Meso- to Neoproterozoic age, is a potential candidate of asteroid impact crater for last many decades. A fresh observation on remote sensing images (ASTER, Landsat and Google Earth Imageries) along with structural analyses in field show that this rectangular structure has a rim-to rim diameter of ~<2.5 km with a present diameter/depth ratio of ~12, a small central conical peak (~6 m high), and quaquaversal dips of rim crest sandstones with average dips between 21° and 50°. Unlike the surrounding sedimentary rocks, which only show two sets of wide-spaced (~2 m) vertical fractures trending NE-SW and NW-SE, the country rocks within the structure show extreme brittle deformation including vertical fractures in numerous directions, moderately dipping fractures trending mostly NE-SW and NW-SE, and moderate fault planes with N-S and E-W trends. The geometry of the Ramgarh structure is very similar to those of asteroid impact craters, where the profound brittle deformation of the sedimentary country rocks within the structure could have been resulted due to sudden shock during the impact. Reactivation of fractures existing within the pre-impact country rocks inside and adjacent to the Ramgarh structure by the shock effect is also possible. Our present observation on sub-samples from a cm-sized glassy silicate piece and our previous study on mm-sized particles, recovered from this structure, show that these magnetic materials have very high Natural Remanent Magnetization (NRM) (~2–19 Am−1) and NRM to saturation isothermal remanent magnetization ratio (REM) (~7–145%) indicating the presence of a high magnetic field during their formation, much higher than the ambient Earth’s magnetic field. A natural phenomenon that could generate a unique ring-shaped deformation structure on a monotonously flat-lying, undeformed sedimentary country rock as well as a high magnetic field in and around this structure is a hypervelocity asteroid impact. The rectangular shape of the Ramgarh structure, which resembles the Arizona Crater, USA, was resulted due to post-impact dextral slip along a NW-SE unnamed fault, followed by dextral NE-SW faulting and minor sinistral slip along E-W fracture. These fractures reactivated perhaps during the modification stage of evolution of the Ramgarh structure. Our remote sensing observation further confirms that the impact took place on the palaeo-channel of Parvati River, which is now displaced towards W due to impact.

Journal of The Geological Society of India, 2002

A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have in... more A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have intruded the metasedimentary rocks of Bihar mica belt (BMB), representing north-eastern extremity of the Satpura orogenic belt of central and eastern India. The metasedimentary rocks of BMB rest on the Chotanagpur gneissic complex (CGC) with an erosional base and have recorded three phases of folding (F 1 , F 2 and F 3 ), where the first two phases are the most dominant. A re-examination of structural, petrological and geochemical data, as well as study on new Rb-Sr whole rock isotopic ages of the BMB granite plutons, suggest that they are post-orogenic in nature in relation to the evolution of the basement (Chotanagpur gneissic complex). These plutons were emplaced between the second phase of compressive deformation (F 2 ,) of the Satpura orogeny, and before the establishment of post-compressional extension, when the regional stress pattern was likely to be transpressional or transextensi...

Geological Journal, 2021

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal e... more The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in...

Geological Society of America Abstracts with Programs, 2019

Lithos, 2017

One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, S... more One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the ~3.25 Ga (U-Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous condition, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into midcrustal levels (c. 17 km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or+ab)] ~0.48 could be ~500 MPa and close to 1000 o C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg+Fe) ~0.41-0.45] suggests that the final temperature of crystallization of the pluton was close to 800 o C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO 2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzofeldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In this model, post-partial melting magma mixing is perhaps critical in explaining the Daly gap in composition and extreme variations in chemical (e.g., SiO 2) and isotopic compositions observed in many bimodal A-type granite suites. The

Journal of Earth System Science

DHALA IMPACT STRUCTURE, INDIA. Md. Arif, S. K. Patil, J. K. Pati and S. Misra, Birbal Sahni Insti... more DHALA IMPACT STRUCTURE, INDIA. Md. Arif, S. K. Patil, J. K. Pati and S. Misra, Birbal Sahni Institute of Palaeosciences, Lucknow-226007, India (arif@bsip.res.in), Dr. K. S. Krishnan Geomagnetic Research Laboratory, IIG, Allahabad-221505, India, Department of Earth & Planetary Sciences, University of Allahabad, Allahabad-211002, India, SAEES, University of KwaZulu-Natal, Durban-4000, South Africa (misras@ukzn.ac.za).

Journal of the Geological Society of India

ABSTRACT

Meteoritics & planetary science

The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps b... more The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (~65 Ma) at ~570±47 ka ago by an oblique impact of a possible chondritic asteroid that stuck the pre-impact target from the east at an angle of ~30-45o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east-west plane of impact (Misra et al., 2010). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g. low-field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high coercivity and high temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K3 (minimum) AMS axes of shocked basalts from around the cr...

Journal of the Geological Society of India, 2009

... We are indebted to Kailas Sekhar Banerjee, Indian Institute of Technology, Kharagpur, for his... more ... We are indebted to Kailas Sekhar Banerjee, Indian Institute of Technology, Kharagpur, for his excellent help during sampling for this work; to B. Das, P. Guha and BPMirddya, XRD laboratory, CRF, IIT Kharagpur, for their expert guidance during XRD analyses; and to the Head ...

Geological Magazine, 2010

Gabbro samples (c. < 0.4 Ma old) dredged from close to the ‘Vityaz Megamullion’ on the slow-sp... more Gabbro samples (c. < 0.4 Ma old) dredged from close to the ‘Vityaz Megamullion’ on the slow-spreading Northern Central Indian Ridge (NCIR, 18–22 mm yr−1) include mostly olivine gabbro and Fe–Ti oxide gabbro. The cumulate olivine gabbro shows ophitic to subophitic texture with early formed plagioclase crystals in mutual contact with each other, and a narrow range of compositions of olivine (Fo80–81), clinopyroxene (magnesium number: 85–87) and plagioclase (An67–70). This olivine gabbro could be geochemically cogenetic with the evolved oxide gabbro. These gabbro samples are geochemically distinct from the CIR gabbro occurring along the Vema, Argo and Marie Celeste transform faults and can further be discriminated from the associated NCIR basalts by their clinopyroxene (augite in gabbro, and diopsidic in basalts) and olivine (gabbro: Fo80–81, basalts: Fo82–88) compositions. Our major oxide, trace element and REE geochemistry analyses suggest that the gabbro and the NCIR basalts are ...

Nanoparticles formed within the ejecta plume are thought to play an important role in the catastr... more Nanoparticles formed within the ejecta plume are thought to play an important role in the catastrophic events due to meteorite impacts. However, the mode of origin of these nanoparticles in the ejecta fallout is still not clearly known due to its poor preservation potential and alteration. Conclusions The presence of single domain crystalline nanoparticles, i.e. non-biogenic titanomagnetite with distinct hexagonal and octahedral crystal habit, in the ejecta fallout supports its formation due to vapor-solid condensation within the impact ejecta plume and the prevalence of the metallic phases (i.e. Fe and Ti) in the ejecta plume probably facilitated the nano-phase crystallization. Objectives To understand the development of the nano-phase in the ejecta fallout, isolated magnetic particles from the ejecta blanket of the Lonar Crater in India Methods To understand the development of the nano-phase in the ejecta fallout, isolated magnetic particles from the ejecta blanket of the Lonar Crater in India were analyzed using a transmission electron microscope (TEM) and EELS. a high sensitivity scanning electron microscope (SEM) equipped with energy dispersive spectrum analyzer (EDS). Methods Isolated magnetic particles from the ejecta blanket of the Lonar Crater in India were obtained through manual separation of magnetic particles, sieving, washing the less than 62 um size fraction with 7N HCL, followed by sonication and settling in deionized water. The isolated magnetic particles were analyzed using a transmission electron microscope (TEM) and equipped with electron energy loss spectroscope (EELS), to determine the composition of the nanoparticles. TEM analysis was carried out in a Libra 120 Zeiss system operating at 120 kV with a point to point resolution of 0.34 nm. The images were taken using bright field imaging and selective area electron diffraction was performed in an area of 100 nm with high population of nanoparticles.

Journal of the Geological Society of India

Meteoritics & Planetary Science

The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angu... more The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejec...

Tectonics and Structural Geology: Indian Context, 2018

The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-... more The Ramgarh structure of SE Rajasthan, central India, situated within an almost undeformed, flat-lying Vindhyan Supergroup of sedimentary rocks of Meso- to Neoproterozoic age, is a potential candidate of asteroid impact crater for last many decades. A fresh observation on remote sensing images (ASTER, Landsat and Google Earth Imageries) along with structural analyses in field show that this rectangular structure has a rim-to rim diameter of ~<2.5 km with a present diameter/depth ratio of ~12, a small central conical peak (~6 m high), and quaquaversal dips of rim crest sandstones with average dips between 21° and 50°. Unlike the surrounding sedimentary rocks, which only show two sets of wide-spaced (~2 m) vertical fractures trending NE-SW and NW-SE, the country rocks within the structure show extreme brittle deformation including vertical fractures in numerous directions, moderately dipping fractures trending mostly NE-SW and NW-SE, and moderate fault planes with N-S and E-W trends. The geometry of the Ramgarh structure is very similar to those of asteroid impact craters, where the profound brittle deformation of the sedimentary country rocks within the structure could have been resulted due to sudden shock during the impact. Reactivation of fractures existing within the pre-impact country rocks inside and adjacent to the Ramgarh structure by the shock effect is also possible. Our present observation on sub-samples from a cm-sized glassy silicate piece and our previous study on mm-sized particles, recovered from this structure, show that these magnetic materials have very high Natural Remanent Magnetization (NRM) (~2–19 Am−1) and NRM to saturation isothermal remanent magnetization ratio (REM) (~7–145%) indicating the presence of a high magnetic field during their formation, much higher than the ambient Earth’s magnetic field. A natural phenomenon that could generate a unique ring-shaped deformation structure on a monotonously flat-lying, undeformed sedimentary country rock as well as a high magnetic field in and around this structure is a hypervelocity asteroid impact. The rectangular shape of the Ramgarh structure, which resembles the Arizona Crater, USA, was resulted due to post-impact dextral slip along a NW-SE unnamed fault, followed by dextral NE-SW faulting and minor sinistral slip along E-W fracture. These fractures reactivated perhaps during the modification stage of evolution of the Ramgarh structure. Our remote sensing observation further confirms that the impact took place on the palaeo-channel of Parvati River, which is now displaced towards W due to impact.

Journal of The Geological Society of India, 2002

A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have in... more A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have intruded the metasedimentary rocks of Bihar mica belt (BMB), representing north-eastern extremity of the Satpura orogenic belt of central and eastern India. The metasedimentary rocks of BMB rest on the Chotanagpur gneissic complex (CGC) with an erosional base and have recorded three phases of folding (F 1 , F 2 and F 3 ), where the first two phases are the most dominant. A re-examination of structural, petrological and geochemical data, as well as study on new Rb-Sr whole rock isotopic ages of the BMB granite plutons, suggest that they are post-orogenic in nature in relation to the evolution of the basement (Chotanagpur gneissic complex). These plutons were emplaced between the second phase of compressive deformation (F 2 ,) of the Satpura orogeny, and before the establishment of post-compressional extension, when the regional stress pattern was likely to be transpressional or transextensi...

Geological Journal, 2021

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal e... more The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in...

Geological Society of America Abstracts with Programs, 2019

Lithos, 2017

One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, S... more One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the ~3.25 Ga (U-Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous condition, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into midcrustal levels (c. 17 km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or+ab)] ~0.48 could be ~500 MPa and close to 1000 o C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg+Fe) ~0.41-0.45] suggests that the final temperature of crystallization of the pluton was close to 800 o C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO 2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzofeldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In this model, post-partial melting magma mixing is perhaps critical in explaining the Daly gap in composition and extreme variations in chemical (e.g., SiO 2) and isotopic compositions observed in many bimodal A-type granite suites. The