Petrology, Structure, and Evolution of a Precambrian Volcanic and Plutonic Complex, Tonto Basin, Gila County, Arizona

Citation

Conway, Clay Michael (1976) Petrology, Structure, and Evolution of a Precambrian Volcanic and Plutonic Complex, Tonto Basin, Gila County, Arizona. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hz23-6e04. https://resolver.caltech.edu/CaltechTHESIS:02062024-221007119

Abstract

Precambrian exposures in Tonto Basin, central Arizona, are among the best in the southwest and the rocks are superbly preserved. Stratigraphic, structural, and petrologic relations of the Tonto Basin rocks, as determined in this study, contribute to our understanding of an important interval of Precambrian history in the southwest, and to the petrogenesis of volcanic and plutonic rocks emplaced in a great silicic alkali magmatic event.

In the only detailed field study within the Tonto Basin prior to the present work, Gordon Gastil defined a stratigraphic sequence fundamentally eugeosynclinal (wacke, slate) in lower parts becoming more miogeosynclinal upward (conglomerate, sandstone) and culminating with a great thickness of rhyolite. Current mapping shows that the extrusive rhyolite sequence is about 2 km thick and is overlain by a km of quartzite. Through joint efforts of L. T. Silver, K. R. Ludwig and the author, a correlation is apparent with corresponding eugeosynclinal, rhyolite and quartzite sequences in the Mazatzal Mountains. The names Alder Group, Haigler Group, and Mazatzal Group are proposed for the respective sequences in both areas. The Alder Group includes not only Gastil's lowermost Alder Formation (here renamed Breadpan Formation) but all of his overlying formations beneath the Haigler rhyolite. Haigler Group in Tonto Basin is composed of Winter Camp Formation, Haigler rhyolite undivided, and overlying Oxbow Rhyolite. The first was the basal part of Gastil's Haigler Formation, and the Oxbow Rhyolite remains as defined by Gastil. Haigler rhyolite undivided includes most of Gastil's Haigler Formation, much of his Hell's Gate Rhyolite (the remainder is intrusive), and extrusive rhyolite which is wide-spread in Tonto Basin outside Gastil's mapped area. The name Christopher Mountain Quartzite is proposed for quartzite of the Mazatzal Group overlying Haigler Group rhyolites in Tonto Basin.

The folded sedimentary and volcanic strata of these three groups occur in a NE-SW belt flanked on both NW and SE by large granite bodies. The southeasterly granite, near Young, was shown by Gastil to be intrusive into Alder Group strata. The northwesterly granite (Payson Granite) was not mapped by him in detail. He hypothesized that both granites were part of a single widespread batholith and that the Payson Granite was gradational through granophyre and intrusive rhyolite into contemporaneous rhyolite (Haigler Group). L. T. Silver subsequently obtained U-Pb zircon ages of 1730 ± 15 m.y. for the Payson Granite, 1650 ± 15 m.y. for the granite at Young, and 1715 ± 15 m.y. for a rhyolite flow in the Alder Group (Flying W Formation), in apparent disagreement with both aspects of Gastil's hypothesis. This apparently placed Payson Granite in a northern, older regional geochronologic province (volcanic rocks~ 1750 - 1820 m.y.; plutonic rocks~ 1720 - 1760 m.y.) and the granite at Young and associated stratified rocks in a southern, younger regional province (volcanic rocks~ 1700 - 1720 m.y.; plutonic rocks~ 1650 - 1700 m.y.), suggesting that Payson Granite might be part of a basement upon which the volcanic and sedimentary rocks were deposited.

Major findings of the present study relating to this problem are that Payson Granite-granophyre, granophyre-intrusive rhyolite, and intrusive rhyolite..,.e:xtrusive rhyolfte contacts are non-gradational intrusive contacts and that Payson Granite has a smooth upper surface dipping gently and apparently concordantly southward beneath the sedimentary and volcanic strata. Enormous composite sills intruded, each beneath the preceding, in the sequence (1) rhyolite porphyry (Hell's Gate Rhyolite and King Ridge Rhyolite), (2) granophyre (Green Valley Granophyre), and (3) alaskite along the upper Payson Granite surface primarily between granite and Haigler rhyolite but locally into the folded stratified rocks (including Alder and Mazatzal Group rocks).

The sills are widespread along the upper Payson Granite surface and no direct relationship between Payson Granite and the stratified rocks could be determined. However, at Gisela where the felsic sills are locally absent, Payson Granite intrudes a small body of distinctive finegrained rock characterized by megacrysts of plagioclase (mafite porphyry). Another of the small scattered masses of mafite porphyry intrudes Haigler rhyolite. It is hypothesized that all mafite porphyries are a single generation of igneous intrusive rock and that all occurrences are correlative. If this is so, Payson Granite must be younger than Haigler rhyolite. In support of this hypothesis, sedimentary and volcanic rocks of nearby pendants in diorite of the Gibson Complex (a differentiated gabbro-diorite body intruded by Payson Granite) contain lithologies similar to those in the Alder and Haigler Group strata.

From independent structural considerations an intrusive contact relation between Payson Granite and the stratified rocks is preferred over alternative basement and thrust contact hypotheses. This preference is based primarily on the apparent continuity of the sub-planar southwarddipping upper granite surface beneath the Gibson Complex. The intrusive hypothesis is also supported by an apparent distribution of more differentiated granite near the upper contact, suggesting a roof zone.

The hypothesized intrusive relation of the Payson Granite to the stratified rocks provides the first suggestion of a structural relation between rocks of the northern and southern geochronologic provinces and places the Payson Granite, and probably the Gibson Complex, within the interval of a great silicic alkali magmatic event in Tonto Basin, intermediate in age between Haigler rhyolite and the felsic hypabyssal sills. The regional implication is that latest plutonism in the northern province overlapped (along the two-province boundary) with earliest volcanism of the southern province.

All Precambrian X units in Tonto Basin have been folded and/or faulted. Primary textures in massive volcanic and plutonic bodies and quartzite are virtually unmodified and penetrative deformation is present only in less competent strata farthest from the large plutonic bodies. Earliest deformation was large-scale folding on NE-SW axes with shallow plunges. This was followed by thrusting and reverse faulting to the northwest under the same regime of NW-SE compression. These early structures were disrupted by left-lateral strike-slip and east-side-down normal offset on NE-SW to N-S faults. All deformation is probably older than Precambrian Y Apache Group rocks and presumably occurred in the interval 1715-1650 m.y. prior to intrusion of the granite at Young and possibly immediately subsequent to the earlier magmatic activity.

The deformation sequence suggests that Tonto Basin was initially the site of foreland folding and thrusting as the southern, basin crustal block impinged northwestward upon the slightly older proto-cratonic mass. It appears that regional tectonic stresses then shifted to give rise to a left-lateral couple in which the southern block was translated northeastward with a strike-slip system developing along or near the two-province boundary. Similarity of this hypothetical tectonic evolution to some modern tectonic histories may imply similar crustal processes .

Haigler rhyolite and rhyolite, granophyre, and alaskite sills are all silicic alkali rocks of similar and distinctive chemical composition. These leucocratic rocks are generally characterized by late-stage alteration of mafic silicate phases to hematite, and by coarse exsolution and pervasive hematite clouding of feldspars. In rare gray (unoxidized) facies, sodic (?) pyroxene (and amphibole?) is preserved in granophyre and intrusive rhyolite. Biotite in alaskite is poorly preserved. Alteration is attributed to hydrothermal activity associated primarily with intrusion of Green Valley Granophyre. Indications of slight alkali depletions and enrichments in Haigler rhyolite and Green Valley Granophyre, respectively, suggest some alkali exchange during the hydrothermal event. From textural and mineralogical evidence in granophyre, alkali enrichment may have occurred in the magmatic state.

Consideration of normative data and feldspar character of the silicic alkali rocks in light of experimental work on the petrogeny's residua system leads to the interpretations that the magmas were water-undersaturated at the deep site of phenocryst formation and that certain magmas originated by partial melting, The enormous volume of silicic rocks and the near absence of intermediate rocks in Tonto Basin argues against an origin by differentiation and suggests that all magmas formed by partial melting.

In a petrogenetic model based on these interpretations, on evidence for minimum depths of emplacement, and on comparative temperatures of formation (estimated by hypersolvus and subsolvus feldspar characteristics and mafic mineralogy), phenocrysts of the porphyritic rocks formed in an intermediate-level region of magma chambers where the various magmas began to crystallize at different temperatures depending on the degree of water-undersaturation. The Payson Granite magma may have been a high, early member. Upon ascension most magmas became water-saturated and were either extruded as ash-flow tuffs or crystallized as relatively coarse sill rocks (alaskite, granophyre, spherulitic rhyolite). Hottest, driest magmas perhaps did not become water-saturated and were extruded as viscous flows or emplaced as fine-grained porphyritic sills. Extreme textural variations and parallel variations in mineralogy in sills of comparable composition and emplacement level are best explained by variable water content.

The Tonto Basin silicic alkali province is a well-preserved, deeply exposed analog of modern ash-flow tuff caldera complexes, notable in its remarkable similarity, particularly in chemical composition, to the Yellowstone rhyolite plateau of Wyoming. Aspects of the petrogenetic model above are compatible with new evidence for the existence of magmas beneath Yellowstone.

Rhyolite and/or granophyre and granite of other silicic alkali provinces (including Yellowstone) commonly occur bimodally with basalt or with tholeiitic layered gabbro bodies. The predominantly silicic rocks in Tonto Basin are associated with small amounts of mafic volcanic and hypabyssal rocks and the large mafic Gibson Complex is thought to have differentia ted from a basaltic magma. The analogy with other bimodal terranes may break down, however, because the Gibson Complex has a calc-alkaline differentiation trend. The calcic character of the Gibson Complex is compatible with inclusion of the complex as part of the northern older province , but seems incompatible with the hypothesized contemporaneity of the Gibson Complex and the silicic alkali rocks. The apparent anomalous differentiation might be explained by assimilation of water by the magma upon intrusion into water-rich Alder Group sediments.

Anorogenic tectonic settings observed for other silicic alkali provinces imply that the Tonto Basin magmatism occurred as a post-orogenic event subsequent to orogeny of the northern province and prior to that of the southern province, or perhaps in a site of back-arc extension during the magmatic stage of the later orogenic cycle.

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