石英交代长石

D I A G E N E T I C R E P L A C E M E N T OF FELDSPAR BY Q U A R T Z IN T H E U I N T A M O U N T A I N G R O U P , U T A H A N D ITS G E O C H E M I C A L I M P L I C A T I O N S t
C. A. \ V A L L A C E U.S. Geological Survey, 1)enver, Colorado 80223 ABSTRACT: Sandstone of the Precambrian Y Uinta Mountain Group has undergone a diagenesis in which quartz replaced feldspar during lithification. Generally, lithification of sandstone was characterized by sequential precipitation of iron oxide~iron oxide q- quartz~ (luartz-~quartz -F illile~illite. Mierogranular hematite bands mantle detrital quartz and feldspar. Authigenic quartz encloses hematite 1)ands on quartz nuclei and protrudes into adjacent febtspar .~rains across ltreserved feldspar outlines Illite is present as inclusions in anthigenic quartz and illite fills interstices in quartz cement. Feldspar dissolution and its replacement hy quartz occurred simultaneously, lllite formed only after replacement of feldspar by quartz ~as ~ell advanced. I)iagenetic replacement of feldspar by quartz has not been noted as a common authitenic feature, even though the geochemical conditions of dia~zenesis do not seem tllltlsual A geochemical model developed here *o illustrate (he processes of diagenesis in sandstone includes the following elemems, 1 ) the ~ ater initially trapped with the sediment was probably hrackish or similar to normal seawater in composition, 2} probahle maximum burial depth was about 4,8(10 m, 3) the estimated nlaximum paleotemperature was 200°C at the depth (using Gulf ('uast geothermal gradients its an analogue), aml 4) membrane-fihering and intracrystalline water expu]sion from clay dnring llrogressive hurial resulted in profound changes in pore water COml)osition that governed the sequential stability of aulhigenic minerals. l'ublished data on compaction experiments and chemical analyses of modern sandstone-ltorewaters, together with textural evidence from the Uinta Mountain Group, suggest the possihility that adjacent interl)edded shale xxas a major source of dissolved SiO, for quartz l,recipitation in simdstone t)cds, l)issolved SiO._. that was expelled during early compaction e, luilil)rated at saturation aml supersaturation vahles for quartz, and quartz precipitated on (letrital grains in sandslone units. K* and H* increased in I)roportion to other ions in sandatone beds during early lithification, and feldspar became a stable solid phase. As burial continued, K ÷ in sandstone l)ore water decreased and microcline dissolved while quartz continued to precipitate, resulting in quartz replacenlent of feldspar and developnmnt of extensive quartz overgrowths, lllite joined quartz as at stable solid phase in the final stages of lithification diagenesis. Because Mg "+, Ca ++, and CI were trapped in shale beds, the chemicalequilibriuna conditions favored crystallization of chlorite in shale units, but pore-water composition caused illite to he the dominant authigenic clay" mineral in sandstone units. Lithification ~as probably coml)lete by the time the sediment had reached a burial depth of about
composition of f o r m a t i o n water, and depositional e n v i r o n m e n t s . T h e central point o f these r e c o n s t r u c t i o n s is that the sequence in w h i c h authigenic mineral c e m e n t s w e r e f o r m e d is a product o f i n t e r a c t i o n s b e t w e e n pore-fluids with c h a n g i n g composition and solid mineral phases at c e r t a i n t e m p e r a t u r e s and pressures. This paper will p r e s e n t an e x a m p l e of a geochemical model for lithification diagenesis in clastic s e d i m e n t a r y rocks of the P r e c a m b r i a n Y U i n t a M o u n t a i n Group in Utah. H o w e v e r , in this model, i n f o r m a t i o n about g e o t h e r m a l gradients, c h a n g e s in composition of f 4 °
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C. ,4. W A L L A C E be necessary to, 1) describe the general diagexeric features of the Uinta Mountain Group, 2) describe the replacement textures and their mineralogie components, and 3) demonstrate that the replacement process is related to lithification diagenesis rather than to later diagenetic modification. The geochemical model for diagenesis of the Uinta Mountain Group will be developed by estimating the temperature at maximum burial depth, and by estimating change in formation-water composition from experimental sediment-compaction studies, from drill-hole data from younger sedimentary basins, and from theoretical activity diagrams. In the western half of the Uinta Mountains, Utah, the upper 3,500 m of the unmetamorphosed elastic Uinta Mountain Group is exposed in the core of the range. In general, poorly sorted arkose and poorly sorted argillaceous rock are confined to the northern half of the area shown in Figure 1, whereas better sorted subarkose, orthoquartzite, and shale are most common south of the range crest. In the western part of the range, Wallace (1972) divided the exposed 3,500 m of the Uinta Mountain Group into seven formations that form five interrelated facies assemblages. Two small dikes were mapped in the area, but feldspar replacement by quartz does not seem to be related to these intrusions. Feldspar replacement by quartz is, instead, confined to cleanly washed sandstone beds. Basement rocks that unconformably underlie basal Uinta Mountain Group conglomerates have been dated at about 1,520 m.y. and 2,300 m.y. by K - A r and Rb-Sr methods (Hansen, 1965). The Red Pine Shale, the uppermost formation of the Uinta Mountain Group, is older than 950 m.y. according to a preliminary Rb-Sr whole-rock determination (Crittenden and Peterman, 1975).