Petrogenesis of acid rocks of the deccan traps

The acid rocks of the Deccan Traps including microgranites, felsites, rhyolites and related rocks are contined to tectonically weak zones in Western India. The significance of their distribution is discussed and the opinions expressed on the genesis of the rocks are critically reviewed. In their formation, fractional crystallization of tholeiitie basalt magma, which is supposed to be the parental one, was aided in certain localities by the melting of the sialic crust and assimilation or partial melting of the pre-Deccan Trap rocks. Evidence for the latter is found in the gradational contacts of the acid rocks with the earlier rocks, their association with cruptive centres or faulted zones and absence of any definite trend of variation in petrographical and chemical characters. Some of the rhyolitic rocks are also formed due to hydrothermal alteration of sedimentaries along the Narmada Valley.     

Mafic alkalic magmatism in central Kachchh,India: a monogenetic volcanic field in the northwestern Deccan Traps

Magmatism in Kachchh, in the northwestern Deccan continental flood basalt province, is represented not only by typical tholeiitic flows and dikes, but also plug-like bodies, in Mesozoic sandstone, of alkali basalt, basanite, melanephelinite and nephelinite, containing mantle nodules. They form the base of the local Deccan stratigraphy and their volcanological context was poorly understood. Based on new and published field, petrographic and geochemical data, we identify this suite as an eroded monogenetic volcanic field. The plugs are shallow-level intrusions (necks, sills, dikes, sheets, laccoliths); one of them is known to have fed a lava flow. We have found local peperites reflecting mingling between magmas and soft sediment, and the remains of a pyroclastic vent composed of non-bedded lapilli tuff breccia, injected by mafic alkalic dikes. The lapilli tuff matrix contains basaltic fragments, glass shards, and detrital quartz and microcline, with secondary zeolites, and there are abundant lithic blocks of mafic alkalic rocks. We interpret this deposit as a maar-diatreme, formed due to phreatomagmatic explosions and associated wall rock fragmentation and collapse. This is one of few known hydrovolcanic vents in the Deccan Traps. The central Kachchh monogenetic volcanic field has >30 individual structures exposed over an area of ∼1,800 km² and possibly many more if compositionally identical igneous intrusions in northern Kachchh are proven by future dating work to be contemporaneous. The central Kachchh monogenetic volcanic field implies low-degree mantle melting and limited, periodic magma supply. Regional directed extension was absent or at best insignificant during its formation, in contrast to the contemporaneous significant directed extension and vigorous mantle melting under the main area of the Deccan flood basalts. The central Kachchh field demonstrates regional-scale volcanological, compositional, and tectonic variability within flood basalt provinces, and adds the Deccan Traps to the list of such provinces containing monogenetic- and/or hydrovolcanism, namely the Karoo-Ferrar and Emeishan flood basalts, and plateau basalts in Saudi Arabia, Libya, and Patagonia.     

The structural control and genesis of alkaline rocks in central Kenya

In the Nairobi area and the adjacent region of the Gregory Rift Valley of Kenya two series of Cainozoic alkaline volcanic rocks, of mildly and strongly alkaline type respectively, are considered to have been derived from a single parental magma of alkali olivine basalt composition. Vulcanicity is genetically associated with tectonic movements attendant upon doming and rifting and distances from the rift margin decrease with crystal fractionation, the most acid differentiates being located at the maximum elevation of the rift floor. An early period of nepheline-bearing lava extrusion associated with central vulcanism is followed by a later period of welded tuff — trachyte — rhyolite fissure eruption. Caldera formation associated with central vulcanism within the Rift accompanies the later volcanic phase.     

Chemical characteristics of some basaltic rocks of India

An analysis of the chemical characteristics of about 200 basaltic rocks of India indicate that the rocks of Pavagarh, Girnar, Rajmahal, Mundwara, Cuddapah and Panjal Trap form independent magmatic series of mixed type having entirely different chemical characteristics relative to the Deccan basalt. The tholeiitic and olivine or alkaline basalts do not seem to form independent magma types. The present study indicates that the rocks belonging to the above magma types coexist together and may represent differentiates of a single magma series.     

Remobilization of granitoid rocks through mafic recharge: evidence from basalt-trachyte mingling and hybridization in the Manori–Gorai area,Mumbai, Deccan Traps

Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting mafic enclaves crop out in the Manori–Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry. Local hybridization has occurred and has produced intermediate rocks such as a trachyandesitic dyke. Feldspar crystals have complex textures and an unusually wide range in chemical composition. Crystals from the trachytes cover the alkali feldspar compositional range and include plagioclase crystals with anorthite contents up to An₄₇. Crystals from the mafic enclaves are dominated by plagioclase An_72–90, but contain inclusions of orthoclase and other feldspars covering the entire compositional range sampled in the trachytes. Feldspars from the hybridized trachyandesitic dyke yield mineral compositions of An_80–86, An_47–54, Ab_94–99, Or_45–60, and Or_96–98, all sampled within individual phenocrysts. We show that these compositional features are consistent with partial melting of granitoid rocks by influx of mafic magmas, followed by magma mixing and hybridization of the partial melts with the mafic melts, which broadly explains the observed bulk rock major and trace element variations. However, heterogeneities in Pb isotopic compositions of trachytes are observed on the scale of individual outcrops, likely reflecting initial variations in the isotopic compositions of the involved source rocks. The combined data point to one or more shallow-level trachytic magma chambers disturbed by multiple injections of trachytic, porphyritic alkali basaltic, and variably hybridized magmas.     

Palaeomagnetism and the Deccan Trap volcanism

The results of palacomagnetic studies made on the Deccan Traps by various workers are reviewed in the light of the recent palaeomagnetic data on these rocks and the general geological information. It is suggested that: (a) the earlier altitude-polarity classification of the Deccan Traps, suggesting that the flows below the general elevation of 2000±200 feet above mean sea level are of reversed magnetic polarity while those above this horizon are normal, is not without exceptions; (b) the geomagnetic field reversed its polarity several times during the eruption of these lavas; (c) the Deccan Trap eruptions probably consisted of several phases of volcanicity over a protracted period; and (d) the phases of Deccan Trap volcanism, the phases of Himalayan upheaval, and the northward drift of the Indian landmass were rather concrescent events.     

Deccan plume,lithosphere rifting,and volcanism in Kutch,India

Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO₂-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO₂-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.     

Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks

Five minor and trace elements, known to be chemically stable during alteration and metamorphism, have been combined in a set of binary diagrams that distinguish fresh tholeiites from alkali basalts. Of the five elements: Ti, P, Zr, Y, Nb, only P shows slight mobility during metamorphism, which is not sufficient to alter greatly the point distribution on the binary diagrams. Using these stable elements altered basaltic rocks: greenstones, spilites and amphibolites may be distinguished in the same way as fresh basalts, and their original magma may be identified as tholeiitic or alkaline basalt. All five elements are readily and rapidly determined, using XRF, thus this method may be applied as a rapid, easy way of discriminating the magma types of altered basaltic rocks. Using this method it can be demonstrated that alkali basalt magma was produced in minor quantities in the Precambrian.     

The sylhet traps,their tectonic history,and their bearing on problems of indian flood basalt provinces

Recent studies of the Sylhet Traps (? Jurassic) and the overlying Cretaceous-Tertiary sedimentary cover in the southern part of the Khasi Hills, Shillong Plateau in Assam have led to a reconstruction of the tectonic history of the area since Jurassic times; a clear picture regarding the nature of volcanism has also emerged. The history begins with effusion of tholeiitic basalts, apparently through E-W fissures developed in the peneplaned crystalline basement. One of these fractures became a fault (the Raibah fault) along which the northern non-volcanic block moved up relative to the southern block experiencing volcanism. The fault was active during and after the volcanism till Upper Cretaceous times. The sequence of eruption was as follows: (1) tholeiitic basalts, (2) minor alkali basalts (nepheline tephrite), (3) tholeiitic basalts, (4) localised explosive effusion of minor rhyolites and acid tuffs, and (5) tholeiitic basalts. Neither feeder dykes nor volcanic vents have been noted in the Sylhet Traps. There are no agglomerates among the basic flows; the fragmental rocks are actually flow breccias. The formation of the various structures such as flow breccias, layering and flow folds in many of the basalt flows are thought to have been controlled by the angle of slope and the rate of flow. Thus, the Sylhet Trap flood basalts are characterised by quiet effusion through linear fissures. The effusion was followed by a dyke phase, intruding also along E-W fractures, expecially in the monoclinally bent southern portion; the subsequent tectonic history of the area is also characterised by relative uplift and downsinking of different basement blocks. It is concluded that in the Shillong Plateau uparching of the basement led to fracturing, effusion of basalts apparently along some zones of fissuring along which differential vertical movement of basement blocks was taking place. In the light of the foregoing conclusions, available data on the tectonics of the Rajmahal and the Deccan Traps are examined; both these flood basalt provinces have suffered broadly similar tectonic histories as the Sylhet Traps. The various features of flood basalts, viz., large extent, huge thickness, subaerial nature, a post-volcanic dyke phase are interpreted as a consequence of fusion of the Upper Mantle, development of tensional fractures eruptions apparently along fractures between adjoining basement blocks undergoing differential uplift.     

Geological and geophysical investigations on Deccan Traps

Volcanic rocks occupy considerable regions in the western portion of India, attaining a maximum thickness of 7000′ near Igatpuri. These rocks are essentially basaltic in nature and are generally referred to as plateau basalts. An attempt has been made in this paper to present some results of geological and geophysical investigations carried out in the Deccan Traps. Three areas (Ajanta - Long. 75″41′ -75° 45′ E, Lat. 20° 32′ - 20° 35′ 15″ N, 18 sq. miles in area; Ellora -Long. 75″ 11′ - 75° 16′ E, Lat. 20° 1′ - 20° 9′ N, 80 sq. miles in area; and Chincholi - Long. 77° 22′ - 77° 30′ E, Lat. 17° 22′ -17° 30′ N, 50 sq. miles in area) have been chosen for this study because of their geological setting. A large number of field specimens have been collected for petrographic study. This is supplemented by examination of microsections and chemical analyses of a few traps. In the Chincholi area where the trap overlies the granites, limestones seem to intervene in between trap and granites. With a view to estimate the possible thickness of the limestone beds, the distribution of intensity of magnetic field in a portion of the area has been studied with a magnetometer. Magnetic susceptibilities in case of few specimens have also been studied. Elastic constants of Deccan Traps have been determined for fifty specimens, employing the Wedge Method. These are further correlated with textural features and porosity values. Such an integrated geological and geophysical investigation on Deccan Traps is bound to reveal some interesting results.     

Role of water in pantellerite genesis

Sub-solidus, mass transfer experiments along a temperature gradient, in which water reacted with alkali olivine basalt, trachybasalt, and peridotite, have yielded acidic extracts directly from basic material. It is proposed that the process may also occur under magmatic conditions by progressive and efficient enrichment of the upper portions of a magma reservoir in the constituents of petrogeny’s residua via an aqueous fluid. This hypothesis could explain the production of silica-oversaturated rocks from nepheline-normative parent material, and their association with basalts of alkaline affinity but without volumetrically significant compositional intermediates. In the case of the trachybasalt experiment the transfer products were also peralkaline and the hypothesis may thus be extended to peralkaline oversaturated rocks.     

Tertiary spilites and quartz keratophyres of the Wagwater Belt,Jamaica, west Indies

Geochemical and petrographic characteristics are used to identify spilites, quartz keratophyres and potassic quartz keratophyres as the most abundant rock types among the Tertiary volcanics of Jamaica. A few low K₂O dacites occur within the sequence. The rock suite is bimodal and was erupted in a nonorogenic environment, an Eocene rift. The evidence indicates that the keratophyres were formed by alteration of the original dacites during an episode of alkali metasomatism. The concentrations of TiO₂, P₂O₃, Cr, Zr and Y suggest that the spilites were formed from basic rocks similar to the Columbia River or Deccan flood basalts. Their chemical variations indicate that they were formed by processes similar to those leading to the formation of the keratophyres.     

Spherulites and thundereggs from pitchstones of the Deccan Traps: geology,petrochemistry, and emplacement environments

Spherulites and thundereggs are rounded, typically spherical, polycrystalline objects found in glassy silicic rocks. Spherulites are dominantly made up of radiating microscopic fibers of alkali feldspar and a silica mineral (commonly quartz). They form due to heterogeneous nucleation in highly supercooled rhyolitic melts or by devitrification of glass. Associated features are lithophysae (“stone bubbles”), which have an exterior (sometimes concentric shells) of fine quartz and feldspar, and internal cavities left by escaping gas; when filled by secondary silica, these are termed thundereggs. Here, we describe four distinct occurrences of spherulites and thundereggs, in pitchstones (mostly rhyolitic, some trachytic) of the Deccan Traps, India. The thundereggs at one locality were previously misidentified as rhyolitic lava bombs and products of pyroclastic extrusive activity. We have characterized the thundereggs petrographically and geochemically and have determined low contents of magmatic water (0.21–0.38 wt.%) in them using Fourier transform infrared spectroscopy. We consider that the spherulite-bearing outcrops at one of the localities are of lava flows, but the other three represent subvolcanic intrusions. Based on the structural disposition of the Deccan sheet intrusions studied here and considerations of regional geology, we suggest that they are cone sheets emplaced from a plutonic center now submerged beneath the Arabian Sea.     

Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-¹⁸O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.     

Carbon dioxide emissions from Deccan volcanism and a K/T boundary greenhouse effect

A greenhouse warming caused by increased emissions of carbon dioxide from the Deccan Traps volcanism has been suggested as the cause of the terminal Cretaceous extinctions on land and in the sea. We estimate total eruptive and noneruptive CO2 output by the Deccan eruptions (from 6 to 20 x 10(16) moles) over a period of several hundred thousand years based on best estimates of the CO2 weight fraction of the original basalts and basaltic melts, the fraction of CO2 degassed, and the volume of the Deccan Traps eruptions. Results of a model designed to estimate the effects of increased CO2 on climate and ocean chemistry suggest that increases in atmospheric pCO2 due to Deccan Traps CO2 emissions would have been less than 75 ppm, leading to a predicted global warming of less than 1 degree C over several hundred thousand years. We conclude that the direct climate effects of CO2 emissions from the Deccan eruptions would have been too weak to be an important factor in the end-Cretaceous mass extinctions.     

Palaeogeography,geomorphological setting and groundwater possibilities in the Deccan Traps of Western Maharashtra

Deccan Traps are the most extensive geological formations of Deccan Peninsula with the exception of only the metamorphic and igneous complex of Archaean age. Based on their mode of emplacement, geomorphic setting and hydrogeological behaviour over an area of about 5,000 sq. km the authors have classified the Deccan Traps of western Maharashtra into 3 groups, namely, (1) The Deccan Traps of Dhulia district, characterised by numerous dolerite dykes, (2) Areally extensive trap flows of Sholapur and Osmanabad districts resulting from slow and quiescent type of flood eruption occupyng the gently undulating terrain, and (3) the traps of Kolaba, Thana and Bombay-Poona regions characterised by intertrappean sediments, dolerite dykes and volcanic ash beds, indicative of violent outbursts resulting in the Sahyadri geomorphologic unit. The groundwater possibilities in the three groups are to a great extent governed by the nature and constitution of the individual flows. The massive traps with their fracture porosities, the vesicular traps with their minutely interconnected and partly filled vesicles and the intertrappen sediments with their primary porosities play a decisive role in determining the groundwater possibilities in them. In Dhulia district the dolerite dykes to a great extent control the movement of groundwater, and success or otherwise of the well field area depends very much upon its location with reference to adjacent dykes. Areally extensive thick vesicular traps with their gentle dips towards east, in Sholapur district, have to be explored for possible artesian conditions in the downdip directions of the trappean units to be tapped. In the case of Poona, Thana and Kolaba districts, exploratory drilling based on geophysical data (to delineate the nature and extent of water bearing horizons) has to be resorted to. It is, therefore, imperative to sub-divide at this stage Taylor’s Single Unit of Deccan Trap Groundwater Province into 3 Sub-Provinces, based on geomorphological, geological and geohydrological setting in the region of western Maharashtra of the present investigation.     

Geology and geochemistry of the Ol Doinyo Nyokie trachyte ignimbrite vent complex,south Kenya rift valley

The Ol Doinyo Nyokie complex is of late Pleistocene age and occurs in the floor of the south Kenya rift valley. It consists of a shallow depression 5 km long and 3 km wide occupied by ash-flows, surrounded by a zone of trachyte dykes, and with a dome-shaped ignimbrite vent at its eastern end. The complex began to form approximately 0.7 m.y. ago with eruption of ash-flows from fissures accompanied by subsidence, followed by emplacement of dykes in the fissures and the growth of a steep-sided ignimbrite tuff-ring. The rocks are all of quartz trachyte compositions similar to those of the flood lavas upon which the complex is built. Detailed geochemical evidence indicates that the ignimbrite magma was derived from the flood lava magma by alkali feldspar fractionation.     

Deccan Trap and the geologic framework of the Cambay basin

The subsurface information gathered during exploration for oil and gas in the Cambay basin shows it as a deep graben with 5 km or more of Tertiary and Quaternary sediments resting on the Deccan Trap floor. The Trap floor of this graben extends from Lat. 24° N to about Lat. 19° N and possibly further south. The basin is divisible into separate morphotectonic blocks as a result of block differentiation in the Trap basement, reflected in the structural attitudes of the overlying sediments. This differentiation is believed to have originated in the Paleocene. The dominant structural grain of the area to south of the Narmada river is ENE-WSW with block faulting in the Traps along the older Satpura trend. North of the Narmada river, the trend is longitudinal upto the Meshwa river while further north the trends veer to a NNW-SSE alignment. These latter trends, in the greater part of the Cambay basin, were impressed early during its subsidence and are the result of reactivation along the old Dharwarian trends in post-Delhi times. Maximum thickness of the Traps penetrated so far is near Mechsana and Cambay where more than 1000 meters thickness has been drilled through. The drilling and gravity-magnetic evidence shows the thickness of Deccan Traps in this trough to be of the order of 2.5 km and points to the possibility of active subsidence of Cambay basin, concomitant with the outpouring of the basaltic lavas. The age of the Traps in the Cambay basin, as evidenced by the available data, is Upper Crealaceous. The influence of the structural grain of the basaltic floor on the overlying sedimentary sequence is evidenced during all the stages in the evolution of the Cambay Tertiary basin. Conglomerates, wackes and reddish brown clays of exclusive Trap derivation predominate in the sedimentary section in the initial stage of the basin evolution during Paleocene. General absence of well developed terrigenous reservoirs on a regional scale in the Paleogene section is due to predominance of Trap terrain as the provenance of clastic detritus, contributing essentially argillaceous matter.     

Relationships between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the Deccan Traps of the Western Ghats, India

The upper part of the Deccan Traps sequence (Bushe to Mahabaleshwar Formations) shows a statistically significant tendency for the most mafic lavas to be the most contaminated by crustal materials. This is the reverse of the relationship shown by suites evolving by contamination accompanied by fractional crystallisation (AFC). The observed correlations (e.g. between Mg-number and Sr isotope initial ratios) are partly an accidental consequence of the fact that the most mafic lavas are more abundant in the lower part of the sequence, while contaminant availability declines in the upper part. It is probable, however, that the correlations are augmented by increased contamination of hotter magma batches during ascent through dykes, a process during which fractional crystallisation is suppressed by magmatic turbulence. The absence of AFC relationships suggests that most of the contamination took place during the ascent stage rather than in a magma chamber. Other continental flood basalt provinces such as the Parana and Etendeka do show AFC relationships, and it is speculated that this may be a result of magma chamber contamination coupled with flow rates which prevent contamination during ascent.     

Volcán Las Navajas,a Pliocene-Pleistocene trachyte/peralkaline rhyolite volcano in the northwestern Mexican volcanic belt

Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions.