Read Precambrian Geology of the Picuris Range, North Central New Mexico text version

NEW MEXICO GEOLOGICAL

SOCIETY * SEVENTH

FIELD CONFERENCE

PRECAMBRIAN GEOLOGY OF THE PICURISRANGE,

tains.

Overlying lower the quartzite member withapparent conformity theRinconada is schist. Rinconada The schist may be locally subdivided ondalusiteinto biotite hornfels the at bose, staurolite-rich andgneiss, schist quartzite a with distinct local cliff forming andmuscovite-quart-biobed, tite-garnet phyllite thincalcareous with beds. camThe bined thickness these of intervals varies from feetto 800 as muchas 1,850 feetin thePicuris Mountains. Pilar The phyllite which conformably overlies Rinconada the schist consists grayto blackcarbonaceous of quartz-muscovite phyllite a slaty with cleavage is as muchas 2,300 and feet thickness. in

NORTH CENTRALNEW MEXICO

By ¯ Arthur Montgomery Lafayette College, Easton, Pennsylvania

INTRODUCTION ThePicuris Range, prong, a semior is isolated mountainspur Precambrian that of rocks jutswestward the into Rio Grande trough valley or between Espanola Taos, and NewMexico. entire The rangeis in TaosCounty except fora small area thevicinity Dixon, in of which in Rio is Arriba County. areainvestigated The covers approximately 150square miles, most which of lies eastof U. S. HighVadito [ormation.TheVadito formation unconformablyoverlies Ortega the formation hasbeen and clivided way64. by theauthor intothelower conglomerate member the and Theauthor became interested thegeology thisarea upper in of schist member. turn, In thelower conglomerate memwhile wasoperating Harding he the Mineandundertook in berhasbeendivided intobasal quartzite conglomerand with meta-dacites, metaand 1947, 1948, 1949geologic and field workandlaboratory ateinterbedded felsites, andesites, conglomerate quartzite and interbedded with studies permitted preparationBulletin New that the of 30, amphibolites, coarse quartz conglomerate, fine-grainand Mexico Bureau Minesand Mineral of Resources. reThe with scales muscovite. schist of The portwasalsosubmitted November, in 1950, partial in ful- ed quartzite minute member which overlies conglomerate the member withconfillment therequirementsthedegree Doctor of for of of formity hasbeendivided ~ intotwominor units. lower The Philosophy. oneconsists quartz-muscovite interbedded of schist with PRECAMBRIAN ROCKS withflows plagioclase of amphibelites containing and sills ThePrecambrian rocks other thantheEmbudo granite of partly porphyritic plagioclase amphibelites. Overlying aregrouped intotwoformations, older which the of has thisis a sequence quartz-muscovite quartzof schist, beentermed Ortega the formation theyounger and the muscovite~ quartz-biotite and granulite. total The thickVadito formation. turn In eachof these formations has ness theVadito of formation asgreat 4,500 is as feet; the beendivided intomembers in someplaces and the memconglomerate member ranges to 2,000 up feetin thickness, berswerestill further subdivided minor into mappable andtheschist member varies to as muchas 2,500 up feet units, in thickness.

Ortega formation. - TheOrtega formation first was proEmbudo granite. TheEmbudo granite intrudes older the posed Justwhousedit forquarzite is exposed by that in meta-sedimentary in thePicuri Range, theexrocks s and theOrtega Mountains 25 miles some northwest thePic- posed of areas arebelieved be onlyapophyses stock° to and urisRange andacross RioGrande the valley. Bothat the likebodies froma muchlarger underlying granitic mass. The typeareaandin thePicuris Mountains Ortega the formagranite microcline-rich, also is butis relatively in rich tionconsists metaof sedimentary andhasbeendirocks soda. modal The analyses thinsections various of of occurvidedintoseveral members. basal The member a seis rences showthefollowing percentage ranges: quartz, 26quence quartzites hasbeentermed lower of and the quart- 50%;microcline, 20-35%; albite-oligoclase, 26-33%; biozitemember theauthor. consists coarse-grainedrite by It of etc., 10%. 0quartzite thin in beds sillimanite-kyanite It is of gneiss. at least 2,500 feetthick exposed thePicuris as in MounIn addition~ arelimited there exposuresbiotite of granite, gneissic granite, neutral and granite thearea. in ¯ Editor's note: This discussion an abstract Bulis of letin of theState 30 Bureau Mines Mineral of and ReDikes andveins. Pegmatites locally are veryabunsources, Mexico New Institute Mining Techof and dant thePicuris in Mountains, general their distribution nology. abstract prepaired theeditorial The was by being apparently controlledproximity major by to granitic staff withtheAuthor's permission, bodies. pegmatites thePicuris The of Range varyin size

143

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NEW MEXICO

GEOLOGICAL

SOCIETY

* SEVENTH

FIELD CONFERENCE

from thread- veinlet$ large like to dikes several thick feet andhundreds thousands feetin length. or of However, a thickness greater 10 feet unusual, of than is

dular bodies. diabase theusual The has ophitic texture with crisscrossing lathes plagioclase interstitial of and patches coarse of rounded grains augite. of

Pegmatites the Picuris of Rangeareof the common PALEOZOIC AND CENOZOIC ROCKS coarse-grained granitic types withonlya fewexceptions. A lithologic study thePrecambrian in thePicuris of rocks In general theyaresimple compostion, locally in but may Range formed major the partof theinvestigation,inasbut be morecomplex. Coarse aggregates fleshpinkmicro- muchas bothPaleozoic Cenozoic of and strata areexposed dineandwhite palesmoky to quartz usually formabout in theareasome attention given these is to sediments. 90%of themasses. Partly perthltic microcline occurs as block~/masses thecenters locally near and along sides Pennsylvanian the roe'ks of thethicker dikes. Quartz, however, likely occupy is to Folded butrelatively unmetamorphosed Pennsylvanian thecores. Coarse books silvery of grayto grayish green mu: rocks assigned theMagdalena to groupoverlap Precamthe covite appear nearthecenters alsoalong borde÷s brian and the crystalline uncomformably faulted rocks or are against of thepegmatites. to sugary fine-grained Platy and albite these rocks along eastern the boundary thePicuris of Rancommonly present a partial is as replacementthemicro- ge.These of Paleozoic rocks extend along thestructural sag dine. Thinsprinklings tinybrownish spessartite between Picuris of red the Range andthe Sangre Cristo de Mouncrystals alsocommon. number thesmaller are A of pegma- rains fromthelower reaches thecanyon RioRanchos of of riteveins thePicuris in Range contain rough prisms or de Taosacross S. Hillandto thecanyon theRio U. of knobby crystals white of beryl several cehtimeterslarger Pueblo. or Theyconsist chiefly arkose, of quartzite, congloin maximum dimensions. Platycrystals blackcolumbiteof merate, shale, somelimestone. arebelieved and They to tantalite sparingly are present some thepegmatites, be DesMoines age. in of in Cleavelanditea common is associate theberyl of andcolumbite. Harding The pegmatite characterizedimis by Tertiary Quaternary and rocks mense concentrations of lithium, tantalum, beryllium, and Picuris tuf/.Tufts thathavebeenassigned thePito butonlyoneother complex pegmatite known theP;is in curls tuffof Cabot exposed a number areas and are at of in curls Range. Thisoccurrance in thelower is quartzite cf adjacent thePicuris to Range andnotably in Arroyo so theOrtega formation about mileup Fletcher a Canyon in Miranda. thebasethere usually coarse At is a conglomthevicinity Pilar. of Thissmall pegmatite regarded is as erate derived fromPrecambrian Paleozoic and strata above complex because contains it fairly abundant lepidolite in which there thinlayers red, are of yellow, olive-green, or coarse platy aggregates.thesouthern of therange white In part clay. Above thisareboulder bedswitha clayey or coarse blacktourmaline common someof the pegma- tuffaceous is in matrix overlain bedsof coarse, by hard, pinkish rites, israre unknown thenorthern but or in part. volcanic breccia thatis associated softwhite with volcanic breccia wellas withbedsof shale as andcompact marl. Aplite dikes. Aplite dikes stringers cutthe as that Locally there thin are andhighly altered basalt flows. Highgranite someof themetamorphic and rocks arecommon at er inthesection thick are strata gray-white of waterlaid a number places therange. of in Aplites thePicuris of tuffinterbedded coarse finegravel somevery with and and Range arefine-grained pinkto gray-white flesh rocks coarse boulder beds.Toward top,thesequence the conthatformdikes onlya fewcentimetersaverage in thickness, sists pebbles, of cobbles, boulders these and of strata, the Thinsections showthattheyarea mosaic microcline cobbles boulders of and and beingderived largely fromvolcanic quartz, rocks contrast thedominantly in with Precambrian and Paleozoic types nearthebaseof thesequence. PiThe Q~artz veins. Quartz veins arealsoabundant thePi-curls in tuffis estimated be as muchas 1,250 1,750 to to curls Range. Locally these areorebearing contain and tra- feet thickness. in cesof copper, gold, tungsten, lead, silver, bismuth. and Santa [ormation. Fe - Tilted alluvial of thelate beds Diabc~e dikes. A fewscattered diabase dikeslieeast Miocene early or Pliocene Santa formation Fe border much andwest thestrip Picuris that of of tuff crosses range of thewestern of therange. unitis characterized the half The fromPilar Vadito. to Mostof these dikes as muchas are by itsusual palepink buffcolor itsextremely to and fine 50or 75 feetin thickness. material quite The is typical of sandy clayey or texture. layers gravel present Thin of are diabase inasmuch it is coarse-grained, olive as dark green locally, pebbles the frequently consisting volcanic of matto brownish-black in color, exhibits closely spaced, square erial. Santa overlies Picuris The Fe the formation locally, setblocky jointing, breaths and downon weathering noto butelsewhere rests directly thePrecambrian. on 145

NEW MEXICO GEOLOGICAL

SOCIETY * SEVENTH

FIELD

CONFERENCE

Servilleta [orrnation. - Unconformably overlying the both Picuris Santa bedsnorth therange nearly and Fe of are horizonal withintercalated of basalt. beds flows These havebeennamed ServilJeta the formation Butler. by Gravel andbasalt flows thisformation aredownfaulted of also against Santa bedsnorth thenortheast of the Fe of part rangeandnearPilar. NearRinconada Embudo and thick beds buffcolored andclay theServilleta of sand of formationarefaulted against Santa beds. Fe

Twomajor faults across eastcut the weststructure of therange, Pilar-Vadito andthefault the fault paralleling Alamo Canyon. These arebelieved be older to tearfaults, butveryrecent movement occurred has along thePilaroVadito fault. third A major fault occur may along alluvial the areabordering Copper Mountain road. direct No evidence could found it andthere beenno observable be for has dislocation beds in thecaseofthePilarof as Vadito tear fault. ispossible thetwogreat It that faults which for goodevidence exists haveformed may originally very in GEOLOGIC STRUCTURE late Precambrian butevidence support this time, in of is The metasedimentsthePicuris of Range showa general veryindirect. couldhavebeenassociated LaraThey with east-west strike steep and southward as theresult dips midedeformation. faults be observed several Small can at of intense isoclinal folding. major Two anticlines two places therange arebothtransverse Iongituand in and and major synclines amplitudes oneto twomiles ex- dinal. with of are posed. folds The aredoubly plunging, theangles and of ° plunge average from20 to 30 . In addition, there num- Theborder are faults therange normal of are faults along erous small folds several of hundred feetin average ampli- which ancient the crystalline havebeenuplifted rocks to rudethatareassociated themajor with folds. Their plun- their present level high above flanking the Tertiary beds. gesandaxial planes conform those thelarger to of folds. Mostof thisnormal faulting occurred between middle and Thelonger folds, fromnorth south, to havebeennamed the lateTertiary timealthough someof themovement be may Pilar anticline, syncline, Hondo Copper Hillanticline, and Quaternary. Harding syncl ine.

146

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Precambrian Geology of the Picuris Range, North Central New Mexico

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