JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B8, PAGES 17,727-17,735, AUGUST 10, 1996

Late plutons in Yucatan

M.B. Steiner Departmentof Geologyand Geophysics, University of Wyoming,Laramie

J. DouglasWalker IsotopeGeochemistry Laboratory and Department of Geology,University of Kansas,Lawrence

Abstract. U-Pb measurementsof zirconsfrom two compositeplutons in the Maya Mountainsof the YucatanBlock (Belize) give Late Silurianages. Zirconsfrom one of the five compositional phasesof the MountainPine Ridgepluton yield an ageof 418+3.6 Ma. A secondcompositional phasegives a minimumage of 404 Ma, and zirconsfrom a third phase,although plagued with high commonPb, yield agesconsistent with the othertwo. Zirconsfrom one compositional phaseof the Hummingbird-MullinsRiver plutonindicate an ageof about410-420 Ma. These datademonstrate that two of the threeMaya Mountainsplutons residing among the strataof the Late Pennsylvanianthrough Santa Rosa Group are olderthan that sedimentation.Al- thoughthe third plutonwas not dated,both the similarityof sedimentaryfacies patterns adjacent to it to thoseadjacent to oneof the plutonsdated as Late Silurianand a publishedsingle Rb-Sr ageof 428+41 Ma suggestthis third plutonalso was emergent during Santa Rosa deposition. Thusthe new U/Pb datesand otherdata suggestthat all threeMaya Mountainsplutons pre-date Late Carboniferoussedimentation and that noneintrude the SantaRosa Group. Althoughvery uniformages of about230 Ma amongstall plutons,derived from abundantearlier dating by the K-At system,led to the conclusionthat intrusionmostly had occurredin the Late ,the U-Pb ages(obtained from the samesites as the K-Ar dates)demonstrate that the K-Ar agesdo not derivefrom a Late Triassicintrusive episode. The K-At datesprobably are a signatureof the rifting associatedwith Pangeanbreakup and formationof the Gulf of Mexico. In a reconstructed Pangea,the positionof the Maya MountainsLate Silurianplutons suggests that the Late Silurian Acadian-Caledonianorogen of easternNorth America extended through the regionof the future Gulf of Mexico. Finally,the U-Pb agesof the Maya Mountainsplutons are the sameas those of a groupof shockedzircons found in the Chicxulubimpact structure and its falloutlayer. The pres- enceof theseages in bothlocations suggests that the Maya Mountains exposures may be repre- sentativeof thebasement of theYucatan Block, hence of thebasement impacted by the Chicxulubobject.

Introduction and GeologicSetting The three plutons(Figure 1) enclosedby the sedimentary rocksare (1) the MountainPine Ridge (MPR) in the northwest The Maya Motretainsare a horstwhich providesone of the MayaMountains; (2) theHummingbird-Mullins River (HMR) in few exposuresof pre-Cretaceousrocks of the Yucatan,or Maya, the northeast;and (3) the Cockscomband adjacent,probably at- Block. The exposureis a west-southwesttrending synclinorium tached [Batesonand Hall, 1977], Sapote stock (C/S) in the of Permianand sedimentary strata enclosing three southeast.The MPR plutonis a compositeof five bodiesof dif- graniticplutons (Figure 1). The strata,the SantaRosa Group, ferentcompositions: a diorite-granodiorite,granodiorite, quartz consistof sandstones,siltstones, mudstones, and shaleswith monzonite,granite, and muscovitegranite. The HMR pluton somelimestone and conglomerate; volcanic rocks (ca. 300 Ma) consistsof largebodies of two-micagranite adjacent to a large interfingerwith the depositsin someareas [Bateson and Hall, bodyof granodiorite,and a separatelobe of quartzmonzonite. 1977]. Well-preservedfusulinids, mollusks, including mmnon- The Cockscombpluton is a large singleintrusive body of por- ites, and brachiopodsindicate that this sequenceis Late phyriticbiotite granitewith accessorymuscovite [Bateson and Pennsylvanianthrough mid-Permian (late LeonardJan)in age Hall, 1977;Cole and Andrews-Jones, 1979]. [Batesonand Hall, 1977;Ross, 1979]. The SantaRosa Group This investigationwas undertaken to supporta paleomagnetic has been foldedand has experiencedlow graderegional meta- study of the Pangean paleopositionof the Yucatan Block morphism(lower greenschist facies) [Bateson and Hall, 1977]. [Steiner,1996], and in particularto resolvea conflictin existing radiometricages. PublishedRb-Sr and K-Ar agesfrom one of Copyright1996 by the AmericanGeophysical Union. the plutonsdiffer by 100 m.y., while that plutonand the other two all give nearly identical K-Ar ages. We thereforedated Paper number 96JB00174. thesegranitic-composition Maya Mountainsintrusives using the 0148-0227/96/96JB-00174509.00 U-Pb system.

17,727 17,728 STEINER AND WALKER: SILURIAN PLUTONS IN YUCATAN

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Mountain Pine Ridge Batholith

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-,-r"I I I I /...'..' '..: ß .':...'..' '.':" .':..• I •0•%•1••1I I•i'i'.i"11I Ii IiI /. -. ß . ß'....". i'- . i.. ß . ß'....". .i'i."i '....7 i'./• I [x}x}qIntrusive10 km Rocks I llllllll•l•?'(':':: .",'.','."'.'"":'::-",'.",'"'-'".':': :[ I • 89 ø 88 ø 30' Figure1. Locationof theMaya Mountains and U-Pb sampling sites(geologic map modified from Bateson and Hall [1972], reprintedby pernfission).Abbreviations are Y-B, YucatanBlock; CB, ChortisBlock; GB, GuacamalloBridge sample; MR, MullinsRiver sample; PC, PrivassonCreek sample; and WW, Windroadsam- ple.

Previous Work (Table 1), clusteringaround an age of 230 Ma (new decay constant).The uniformityof K-Ar agesfrom all of theplutonic Eachof the threeplutons has had multipleradiometric age rocksof the Maya Mountainsis notable,as is the 100-m.y. determinationsby eitherone or two methods.Initially, the MPR differencebetween the K-Ar and Rb-Sr ages in thestone body. plutonwas datedby Rb-Sr, givingages of 280-300 and 390 Ma In reconstructingMaya Mountains geologic history, attention [Bateson,1972' Kesler et al., 1974]. Later, a Rb-Sr whole-rock hasfocused mainly on the Late Triassic K-Ar ages, principally isochronage of 336+10 Ma was obtained[Bateson and Hall, becausethey are souniform; Bateson and Hall [1977,p. 42] 1977,recalculated with the newdecay constant by Dawe, 1984], consideredthe uniformityas "primafacie evidence" of a Triassic but the scatterin the dataand the factthat independent analyses intrusiveage for the two easternplutons. However, the eastern of K-feldsparsplotted below the isochronindicated that the plutonshave been dated only by the K-Ar method,with the ex- Rb-Sr systemhad been disturbed by a later event[Bateson and ceptionof oneRb-Sr determination from the Sapotestock, a Hall, 1977].Numerous K-Ar ages(biotite) also were determined smallintrusive body separated from the Cockscomb (C/S) pluton for this pluton[Cole and Andrews-Jones, 1979], as well as for by 10km andthought to beattached at depth. Batesonand Hall the other two plutons,the Sapotestock [Batesonand Hall, [1977]reported a singleRb-Sr analysis with an ageof 428+_41 1977], and one locality in the Bladen Volcanics[Cole and Ma for the Sapotestock, but they dismissedthis age as Andrews-Jones,1979]. All K-Ar agesare remarkablyuniform anomalousbecause their K-Ar agefor this body(232+5 Ma) STEINER AND WALKER: SILURIAN PLUTONS IN YUCATAN 17,729

Table 1. K/Ar Agesof Maya MountainsIgneous Rocks

Age as Recalculated Material IgneousBody Published,Ma Age Dated Reference

MountainPine Ridge 201 +8 (205) * MountainPine Ridge 210 +8 (214) * 2 MountainPine Ridge 228 +8 (232) * 2 MountainPine Ridge 227 +8 (231) * 2

Hununingbird 227 +7 231 +7 biotite 1,3 Hununingbird 223 +7 227 +7 biotite 1,3

Cockscomb 222 +7 226 +7 biotite 1,3 Sapote 232 +5 237 +5 biotite 1,3 Cockscomb 226 +8 (230) * 2,3

BladenVolcanics 232 +8 (236) * 2

Referencesare 1, Bateson& Hall [1977] (recalculatedby Dawe [1984]); 2, Cole and Andrews- Jones[1979]; 3, Dawe [ 1984]. (Not all datawere available for recalculation,therefore extrapolations of thesedata to thenew decay constant, based on the changesin the agesthat were recalculated, are shown in theparentheses.) Asteriskdenotes that type of materialdated was not given.

agreedso well with the K-Ar agesof the adjacentCockscomb of the sedimentaryrocks. The commongeologic scenario for the plutonand the HMR plutonto the north.The SilurianRb-Sr age Maya Mountainsconsists of Penno-Carboniferoussedimentation alsowas rejected because no otherSilurian- rock ages accompaniedby limitedvolcanism, followed by an episodeof were known in Belize [Bateson_andHall, 1977; Cole and folding,and finally,Late Triassicgranitic intrusion [Kesler et Andrews-Jones,1979]. The presenceof the sameK-Ar agesin al., 1974; Batesonand Hall, 1977; Cole and Andrews-Jones, the western,MPR pluton as thosein the easternHMR and C/S 1979;Dawe, 1984;Donnelly et al., 1990]. The timing of the plutonsand the discrepancybetween those K-Ar agesand the folding was bracketedby the youngestfaunal ages of the Rb-Srages of the MPR andC/S plutonsstrongly suggests that at sedimentaryrocks (mid-Pemfian)and the Late TriassicK-Ar least two of the Maya Mountainsplutons may be older than agesof the granites.Folding thus was thoughtto be of Late indicatedby the K-Ar data. Permian-EarlyTriassic age; in addition,this age appeared to be In additionto the Late Triassicages, thermal alteration of the supportedby the presenceof an orogenicepisode of that age metasedimentaryrocks around the marginsof the easternplutons whichdeformed a similarsedimentary sequence in Guatemala hadbeen observed [Cheer, 1928; Dixon, 1956;Bateson and Hall, androcks elsewhere in Centralganerica [e.g., McBirney, 1963' 1977]. The alterationwas confinedto the sedimentaryrocks in Dengoand Bohnenberger, 1969]. contactwith plutonmargins, superposed over the weak regional The belief that intrusionwas a postfoldingevent also was metamorphism.The altered zones were observedto contain inferredto be supportedby thefact that the folding deformation prominentchiastolite (andalusite) [Ower, 1928; Dixon, 1956; doesnot extendinto the plutons[Bateson, 1972]. However,a Batesonand Hall, 1977]. The combinationof observationswere tremendousdifference in rock competencyexists between the interpreted to indicate that the alteration was a contact very fine grained,mostly pelitic SantaRosa sediments and the metamorphismthat wasrelated to plutonintrusion [Ower, 1928; massive,medium- to coarse-grainedgranitic plutons. As recently Dixon, 1956;Bateson and Hall, 1977]. The K-Ar agesled to the demonstratedby Patersonand Tobisch[1988], plutonic rocks do interpretationthat intrusionhad occurredin the Late Triassic notnecessarily record the obvious deformation displayed by sur- [e.g.,Bateson and Hall, 1977]. roundingsedimentary rocks, thus invalidating any support for Studiesinitially reportedthe presenceof contactmetamor- the relativeages of deformationand intrusionin the Maya phism around all three plutons [Ower, 1928; Dixon, 1956; Mounains based on absenceof igneousrock defomlation. Batesonand Hall, 1977], however,a later studyindicated that The mostrecent work in the MPR plutoninferred that all five the alterationaround the western,MPR pluton was a hydro- of the compositionalphases of the westernpluton were intruded thermalin origin [Dawe, 1984]. This hydrothermalalteration prior to SantaRosa sedimentation,at 336 Ma [Dawe, 1984], was observedat the pluton margins,both in the surrounding with subsequentdeposition of the Pennsylvanian-Permianstrata sedimentarysequence and alongjoints and fractureswithin the unconformablyon these intrusives.In Dawe's interpretation, pluton[Dawe, 1984]. This studythus implicitly restrictedto the afterthe periodof regionalfolding and metamorphism sometime easterntwo plutonic bodies,the contactmetamorphism inter- betweenmid-Permian and Late Triassic,the MPR pluton pretedas related to intrusion. experiencedan episodeof hydrothermalalteration and dike Initially, the combination of the apparent contact emplacementduring the Late Triassic.He hypothesizedthat metamorphismand the K-Ar ages led Maya Mountains these events were related to the Late Triassic intrusion of the investigatorsto concludethat emplacementof all plutonshad two easternMaya Mountainsplutons [Dawe, 1984; Donnelly et occurredin the Late Triassic, but later the conceptof Late al., 1990]. Triassic intrusionwas restrictedto the easternplutons and a Subsequentto Late Triassictime, the geologichistory of the portionof the western [Kesler et al., 1971; Hall and Bateson, MayaMountains was relatively simple. The regionsubsided and 1972; Bateson and Hall, 1977; Shipley, 1978]. Generally, was blanketed by Mesozoic and Cenozoic sediments.Late investigatorshave consideredthat this intrusive event was -earliestCretaceous continental sediments (the Todos coevalwith or postdatedthe foldingand regional metamorphism Santos Formation) were depositedon the deforn•edSanta Rosa 17,730 STEINER AND WALKER: SILURIAN PLUTONSIN YUCATAN

Group,followed by a thicksequence of Cretaceousand Tertiary Discussion marine carbonates.Later uplift of the southernend of the Yucatan Block exposedthe older rocks froin beneaththe GeologicRelationship of Plutons to Santa Rosa Group Cretaceous-Tertiarycover. The Late Silurianages for threeof the five compositional GeochronologicResults phasesof the MPR plutonand for the Mullins River bodyof the HMR plutonindicate that at leasttwo of the threeplutons of the Sampleswere collectedfrom threedifferent compositional Maya Mountainspredate deposition of the sedimentaryrocks of phasesof theMPR plutonand from one phase of theHMR plu- the Penno-CarboniferousSanta Rosa Group. ton (Figure lb). MPR sites includeda porphyriticgranite Theseages suggest that the plutonsmay havebeen emergent sampledat GuacamalloBridge on theMacal River, a dioriticto at the time of sedimentation.In the MountainPine Ridge area, granodioriticcoinposition pluton called the WindwardRoad this inferenceis consistentwith the strikingabundance of con- pluton,and a granitesampled at PrivassonCreek. The HMR glomerateand sandstonein the SantaRosa Group near the MPR plutonwas sampledin the Mullins River graniteand granite pluton,in contrastto a generalpaucity of coarse-grainedrocks in porphyryalong the Mullins River. Samples were taken from the mostof the SantaRosa Group of the Maya Mountains[Dixon, samelocalities froIn whichpublished K-Ar ageshad beendeter- 1956; Simmons,1972; Batesonand Hall, 1977; Shipley, 1978]. mined.Samples were sledge-hammered froIn river bed and creek Furthermore,although the SantaRosa Group is dominantlygray exposures.Despite the tropicalweathering envirolm•ent, the in color,red sedimentaryrocks reminiscent of typical"red beds" sampleswere quite fresh,a resultof the activeerosion in the are abundantaround the southernand easternlnargins of this MayaMountains. Abundant zircons were recovered from all four pluton[Simmons, 1972; Steiner, 1996]; the presenceof red beds samplesand were divided into fractionsusing magnetic aroundthe flanks of this pluton is readily consistentwith sub- susceptibility,size, and morphology (Table 2). aerial weathering of an emergent granitic pluton in near- equatoriallatitudes and subaerialor subaqueousdeposition of MPR Pluton the resultant materials adjacent to the pluton, while the predominantlygray Santa Rosa strata were being deposited further offshore. The U-Pb isotopicbehavior of the zirconpopulation froin the GuacamalloBridge site is simple.The zirconsfrom the porphy- Althoughour radiometricdata only deInonstratethat two of ritic graniteappear to havesuffered simple Pb loss,as U-Pb data the threeMaya Mountainsintrusive bodies predate Peimsylva- fromthree fractions give a lineararray of discordantpoints (Fig- nian-PenniansediInentation, published geologic data suggest ure 2a). Pb-Pbages for all fractionsare the samewithin error. that the third intrusive(C/S pluton) also predatesSanta Rosa We interpretthe upperintercept age of 418+3.6Ma to be the age Groupdeposition. The lithologyof the SantaRosa Group departs of crystallizationof thissample [Ludwig, 1990, Model 1]. froIn a doIninantlymudstone-shale grain size primarily in two U-Pb dataof zirconfractions froIn the WindwardRoad grano- areasof the Maya Mountains:one area is aroundthe MPR plu- dioritetrend toward older ages away from concordia, in a pattern ton andthe otheris surroundingthe C/S pluton[see Bateson and consistentwith the incorporationof older, xenocrysticzircon Hall, 1975]. Sandstoneassociated with soIneconglomerate is (Figure2b). The resultsof threebulk fractionsgive a lowerin- abundantnorth and south of the C/S pluton [Dixon, 1956; terceptage of 403 Ma. To test for the effectsof Pb loss,two Batesonand Hall, 1975, 1977]; furthermore,an unpublishedre- handpickedfractions were air abraded.One of the pointslies on port [Muncaster,1976] describedsome red sedimentaryfacies to the previouslydefined chord, while the other falls below the the southof the C/S plutoniccoInplex. chord.We interpretthese results to indicatethat the lowerinter- The radioInetricage determinationsfrom the MPR pluton cept age of 404+3.3 Ma (definedon the four colinearpoints) confirmthe implicationfroIn the coarsersediInentary facies ad- [Ludwig,1990, Model 1] representsthe crystallizationage of jacent to it that this pluton was eInergentduring Santa Rosa thissample. We considerthis a mininmmage, however, because deposition;the similar facies pattern aroundthe C/S pluton it is possiblethat the fractionssuffered bulk Pb lossrather than likewise suggeststhat it also may have been emergent.More- lossmerely confined to theouternmst portions of thegrains. over, this inferencederives supportfroIn the single non-K/Ar Resultsfrom the third site in the MPR pluton,the Privasson radioInetricdate associatedwith this pluton, the Rb-Sr age of Creekgranite, are uncertaindue to the high concentrationof 428+_41Ma froIn the Sapote Stock [Batesonand Hall, 1977]. commonPb in thezircons (Table 2). The2ø6pb/238U agesare in Althoughthis date was discardedat the time becauseof lack of broadagreement, however, with the other results from this plu- any otherevidence of Silurianage rocks in the Maya Mountains, ton (Table 2). it certainlyagrees with our Late Silurianages for the MPR and MR plutons,suggesting that this agemay be morereasonable HMR Pluton than has been thought.Therefore, albeit tentatively,evidence froIn two independentsources suggests that the third Maya The MullinsRiver dataare Inore poorly constrained than that Mountains pluton (C/S) also Inay be older than the Late of the MPR pluton,but are consistentwith theIn.Five zircon -EarlyPermian depositionin this area. Thus, it andthree Inonazite fractions were analyzed.It is apparent appearsthat all three Maya Mountainsintrusives may predate froIn the data that both Pb lossand an inheritedcomponent are the enclosingsedimentary strata. presentin the zirconsfroIn this rock(Figure 2c). A chordfroin 410 Ma to 1600 Ma is shown for reference. Monazites from the The 230 Ma Event saInpleshow someevidence for Pb loss in that they scatter above,below, and along concordia. We takethe ageof 420 Ma, Our agesfor the MPR pluton refine existingdata [Bateson correspondingto the age of the oldestmonazite grain, as the best and Hall, 1977], and the similarityof the agesfroIn the different estimateof the age of this saInple.This age from the Mullins coinpositionalphases of the MPR pluton imply that all of the River bodyof the HMR plutonis entirelyconsistent with the Inultiphasebody was intrudedin the Late Silurianand none of zircondata and ages of theMPR pluton(Figures 2a, 2b). the phaseswere intrudedin Late Triassictime. Our data also STEINER AND WALKER: SILLJ-RIANPLUTONS IN YUCATAN 17,731

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a) the 230-Ma eventprobably was not an intrusiveepisode. This 0.068 evidenceand the uniformityof the K-Ar ages throughoutthe GuacamalloBridge Granite / Maya Mountainslead us to concludethat all the 230-Ma agesof 0.067 the Maya Mountainsrepresent a resetting/coolingevent and not an intrusiveepisode.

co 0.066- 410 /(/ - The 230-Ma eventrecorded in the igneousrocks most likely representsthe breakupof Pangeaand initial rifting that formed

•o 0.065- the Gulf of Mexico. CurrentPangean reconstructions place the YucatanBlock within the Gulf of Mexico, adjacentto southern NorthAmerica and northern South America [e.g., Pindell, 1985; 0.064 - Schoutenand Klitgord, 1994]. The rifting whichseparated Noah // Interceptsat_ America from Africa duringthe breakupof Pangeais aboutthe 390 / ./' 418 _+3.6 and 16 +_95 Ma - same age as the 230-Ma event in Belize, becausethe oldest • /// (MSWD=0.6) sedimentaryrocks filling the rift basins of eastern North I// I I I I 0.0631 America are Camian in age [Comet, 1993]; by current 0.47 0.48 0.49 0.50 0.51 timescales,230 Ma is about the middle of the Carnian[e.g., 207pb/235U Hadand et al., 1990]. Sincethe Late Triassicages in two of the

I I I Maya Mountainsplutons do not representcrystallization ages, b) and sincethe Maya Mountainsand YucatanBlock are thoughtto Windward Road 450 have been locatedin the former regionof the Gulf of Mexico 0.072 _ -- prior to Pangeanbreakup, it is logical that thoseK-Ar ages reflectheating associ-ated with Pangeanrifling. 440 /// / aa / In the MPR pluton,the Late Triassicages have been linked to co• 0.070 the episodeof hydrothermalactivity [Dawe, 1984]; this associa- / - tion and the occurrenceof Late TriassicK-Ar agesthroughout the Maya Mountainssuggests that the hydrothermalevent could • 0.068- 42 have been more widespreadthan just the vicinity of the MPR plutonand may constitutethe 230-Ma thermalevent.

0.066 _ 410 . 1210_136and404+3.3Ma Origin of the Contact Metamorphism (MSWD = 1.0) /// Interceptsat A hydrothermalorigin for the 230-Ma eventwould resolve a 0.50 0.52 0.54 0.56 dilemma which is posedby the presenceof apparentcontact 207pb/235U metamorphosedsedimentary rocks in direct contactwith the granitewe sampledin the Mullins River plutonand for which [ I I I I c) we obtainedobtained a Late Silurian U-Pb age. The adjacent sedimentary rocks clearly are described as contact metamorphosed[Ower, 1928; Dixon, 1956; Batesonand Hall, 0.09 -- MullinsRiver 560• _ 1975, 1977]; those sedimentary rocks were sampled for paleomagneticstudy [Steiner, 1996] at the same time as the radiometricsamples were collectedand definitely appearto be 0.08 more altered than other Santa Rosa strata sampled for paleomagneticinvestigation. 440.// 0.07 _ m•/,•./• aa - The presenceof Late Siluriangranite in contactwith contact- metamorphosedPe•msylvanian-Pemfian sedimentary rocks 4oo/aa necessitates a reconsideration of the nature of the alteration of 0.06 the sedimentaryrocks at thesepluton margins. Reconsideration of the origin of the alterationalso is requiredby the strong - 36//•m - probabilitythat all Maya Mountainsplutons are older than the 0.4 0.5 0.6 0.7 0.8 enclosingsedimentary strata. Three explanationsare possible: (1) The sedimentsaround the eastem plutons are contact 207pb/235U metamorphosedPexmsylvanian-Pennian Smita Rosa Groupand Figure2. Concordiadiagrams for U-Pb analyses.All fits are the U-Pb (HMR pluton)and the singleRb-Sr (C/S pluton)dates model1 of Ludwig[1990]. MSWD denotesmean square of are incorrect;(2) the U-Pb date of the HMR pluton is correct and the pluton is older than Santa Rosa sedimentation,but the weighteddeviates. sedimentaryrocks adjacent to the pluton are not the Pennsylvanian-PermianSanta Rosa Group but an earlier showthat at leasta portionof the HMR plutonwas not intruded sedimentarysequence which was contact metamorphosed during in Late Triassictime as is widely believed[Bateson and Hall, Late Silurian intrusionof the pluton into that sedimentary 1977; Shipley, 1978; Dawe, 1984; Donnelly et al., 1990]. Be- sequence;or (3) the thermal alterationis not contactmeta- causethe localitiessampled for U-Pb dating in the MPR and morphismbut the resultof a hydrothermalevent whose fluids HMR plutonswere the samelocalities from which the 230-Ma werechanneled along the plutonic/sedimentary contacts. K-Ar ageshad been obtained,it is thereforeevident that these The possibilitythat the ËMR date is grosslyin errordue to Late Triassicages do not representcrystallization ages, and that Pb lossor inheritanceis unlikely.Although both the monazite STEINER AND WALKER: SILURIAN PLUTONS IN YUCATAN 17,733 and zircon fractionsindicate complexU-Pb systematics,the and the uniformityof the 230 Ma K-Ar agessuggests the occur- three monazite fractionscluster around concordiaat approxi- renceof a widespreadhydrothermal event in the Maya Moun- mately400 Ma; theymay reflect varying degrees of youngerPb tains. loss.The five zirconfractions yield a nonlineararray that trends Finally, the densityand textural contrastsbetween the fine- from 1600 to 410 Ma and are consistent with a mixture of nfi•er- grained,low-porosity pelitic SantaRosa sedimentsand the me- ited zirconsand thoseibnned during crystallization of the pluton dium to coarsegrained granitic plutonic rocks reasonably could at about410 Ma to 420 Ma (oldestlnonazite). Hence, both min- have causedthe plutonxnargins to serveas conduitsfor hydro- eral systemsonly indicate ages consistent with an agenear 410- thermalfluids. If the contactmetamorphism of the easternplu- 420 Ma, not the 230 Ma agethat hasbeen associated with the tons is a hydrothermalalteration, the mineralogicalditl•rences contactmetamorphism. betweenit andthe marginalalteration around the westernpluton The secondpossibility, that the contact-alteredsedimentary possiblymight be relatedto a differencein structurallevel. The rockis not the SantaRosa Group but an oldersedimentary rock westerlydip of the SantaRosa synclinorium [e.g., Bateson and contactmetamorphosed during intrusion at an earliertime, can- Hall, 1977] may suggestthat the Maya Mountainsblock has notbe ruledout, particularly because detailed descriptions of the beentilted to the west,thus exposing deeper crustal levels in the contactmetamorphosed rocks are not givenin the literature,and eastand a highertemperature regime of the hydrothermalactiv- no similar Santa Rosa strata were sampledin the recmmais- ity. sancepaleomagnetic study [Steiner, 1996]. The chiastolitein the contact-metamorphosedrocks is reportedas altered [Dixon, Relationship to Other Age Data 1956;Bateson, 1972],a fact which might be viewedas support for the conceptthat thesesediments are pre-SantaRosa Group; Only a few radiometric data previously existed for the however,the multipleevents in the geologichistory of this area Yucatan Block or Central America as a whole. The new U-Pb permit numerousopportunities for that alteration to have ages thus significantlyenhance the available data set. Despite occurred. the paucityof data, an age similarto our ageswas reportedfrom The third possibility,that the alterationresults from a Late the northernYucatan Block. A well, Yucatan-l, coredtlu:ough Triassic hydrothermalevent, also is permissible.Two items Tetiary and Cretaceousrocks, terminated in a rock described were the major evidencethat the alterationaround the eastern variouslyas "rhyolite"or "rhyoliteporphyry"; Rb-Sr measure- plutonswas contactmetamorphism: one was its confinementto ments on the rhyolite suggestedan age of 410 Ma [Lopez- the contactsand the other was the presenceof chiastolite Ramos,1975]. This Rb-Sr age is similarto the 428+_41Ma Rb- (andalusite)[Dixon, 1956; Batesonand Hall, 1977]. Abundant Sr age from the Maya MountainsSapote stock, as well as to our tourmaline also had been observed in the thermal aureoles 400420 Ma U-Pb ages. [Batesonand Hall, 1977]. A strict interpretationof the Previously,the Early CarboniferousRb-Sr age for the MPR mineralogyof this contactalteration allows the possibilityof pluton[Bateson and Hall, 1977] had appearedto be supported eithera contact-metamorphicor a hydrothermalorigin. by a similar U-Pb date from the Rabinal granite in adjacent In addition to its formation within contact-metamorphicGuatemala. The Rabinal granite resides among sedimentary pressure/temperatureregime, andalusitealso can be generated rocksof similar lithologiesas the SantaRosa sedimentsof the underthe hydrothermalconditions as well [Roseand Butt, 1979; Maya Mountains[McBirney, 1963], and a U-Pb dateof 345+20 Barnes, 1979]. Therefore, the presenceof andalusite,while Ma had been obtained[Goreberg et al., 1968]. However,the highly suggestive,is not unequivocalevidence of contact Late Silurian U-Pb datesfor the MPR from the presentstudy metamorphism.Furthermore, tourmaline is a commonhydrother- indicatethat the similarityof ageswas fortuitousand a function mal mineral; it was the principalindicator of the presenceof of a disturbedRb-Sr systemin the MPR granite,as Bateson and hydrothermalalteration around the northwestpluton (observed Hall [1977] had surmised. both in hydrothermallyaltered plutonic as well as sedimentary The Late SilurianU-Pb agesfrom the Maya Mountainsplu- rocks) [Dawe, 1984]. The fact that both the hypothesized tonsare similarto thoseof a smallpopulation of shockedzircons contact-metamorphicand the hydrothermalalterations share an associatedwith the Chicxulubimpact structure. The K/T bound- abundanceof authigenictourmaline suggests the possibilityof a ary Chicxulubimpact structure is locatedon the northernend of commonorigin. the YucatanBlock. Shockedzircons from boththe impactbrec- Also, the patternsof the alterationaround the easternplutons cia within the Chicxulubstructure and from the falloutlayer in and the western pluton are similar. Like the hypothesized North America and Haiti (the K/T boundaryclay) show two contact-metamorphicalteration eastern plutons, the hydro- groupsof ages:The majorityof the zirconsare 544.5+5 Ma, but thermalalteration around the westernpluton also is confinedto a populationof 418+6 Ma alsois present[Krogh et al., 1993]. the marginsof the pluton[Dawe, 1984]. Hencethe main differ- The >_404-418Ma plutonages of the presentstudy indicate that, encebetween the easternand western contact alterations appears althoughthe Maya Mountainsare 460 km from Chicxulub,Late to be the presenceand abundanceof andalusitearound the Silurianplutons may be a significantcomponent of the basement easternplutons and not the western.The fact that andalusitecan of the Yucatan Block. Plutons coeval with those dated in the be formedunder hydrothermal conditions allows the possibility Maya Mountains provide an obvious sourcefor the 418-Ma that the easterncontact metamorphism also might be a hydro- shockedzircons associated with the Chicxulubimpact. Hence thermal alteration, perhaps generatedat higher temperature our Maya Mountains radiometricdata add to the little that is conditions.Moreover, hydrothermal alteration is not restrictedto known of the basementtarget rocks impacted at the Chicxulub the MPR pluton;Cole and Andrews-Jones[1979] reportednu- site. merousobservations of hydrothermalalteration in association Finally,the Late Silurianage of the Maya Mountainsplutons with the southernvolcanic complex and summarizedother such is the sameas the age associatedwith the Acadianorogeny of observationsby Flanders[ 1978],Thoreson [ 1980],and Dt•tecker eastern North America (and the Caledonian and Scandian [1978]. Furthermore,the 230 Ma K-Ar datesalso have been orogeniesof Europe). During the existenceof Pangea,the reportedfrom that volcaniccomplex. All of theseconsiderations Yucatan Block has been postulatedto have been locatedjust 17,734 STEINER AND WALKER: SILURI• PLUTONS IN YUCATAN

southof the Ouachitaorogenic belt [e.g.,Pindell, 1985;Pindell provedthe concepts presented in thismanuscript and are gratefullyaclmowl- and Barrett, 1990; Schouten and Klitgord, 1994]. These edged.The BelizeMinistry of Geologyand its director,Evadine Garcia, were reconstructionslocate the Maya Mountainsplutons to the south especiallyhelpful with the logisticsof thisstudy and generous with the Min- istryfacilities, for bothof whichwe aredeeply grateful. of Texas-Louisiana.The similarity of the 400-420 Ma ages of this study to the numerouscirca 400-415 Ma ages of the Carolinas [e.g., Kish, 1990] suggeststhe possibility of a References continuationof the southernAppalachian Acadian-age orogeny along the southernborder of the North ganericancraton. The Barnes,H.L.,, Geochemistryof HydrothermalOre Deposits,2nd ed., JohnWiley, New York, p. 175, 1979. Late Silurianages could indicate either that the Maya Mountains Bateson,J.H., New interpretationof geologyof Maya Mountains,British and Yucatan Block were part of the Africa-SouthAmerica Honduras,Am. Assoc.Pet. Geol.Bull., 56, 956-963, 1972. continentalmass prior to Pangeanbreakup or that the trend of Bateson,J.H., andI.H.S. Hall, Geologicalmap of theMaya Mountains, the Acadian orogenycontinues into South America, perhaps Belize,Nat. Environ.Res. Counc., Her Majesty'sOrd. Surv.,London, 1975. alonga trend similar to that suggestedfor the earlier Taconic Bateson,J.H., and I.H.S. Hall, The Geologyof the Maya Mountains, Belize,Inst. Geol.Sci. OverseasMere., 3, Her Majesty'sStationery Office, orogeny[Dala-Salada, et al., 1992]. London,42 pp., 1977. Cole, G.L., and D.g_ Andrews-Jones,Geological and economicevalua- Summary and Conclusions tionincluding exploration results of the 1978field season, Belize Ann. Rep., 4, 37 p., BelizeMinist. of Geol.,Belmopan, 1979. Late Silurian U-Pb ages were obtainedfrom t•vo the three Comet,B., Applicationsand limitations of palynologyin age,climatic, Maya Mountainsplutons. Both give ageswhich predatethe en- andpaleoenvironmental analyses of Triassicsequences in North America,N. closingLate Pennsylvanianto mid-PermianSanta Rosa sedimen- M. Mus. Nat. Hist. and Sci. Bull., 3, 75-93, 1993. tary strata,previously thought to have been intrudedby the plu- Dala-Salda,L.H., I.W.D. Dalziel, C.A. Cingolani,and R. Varela,Did theTaconic Appalachians continue into southern South America?, Geology, tonic rocksof the Maya Mountains.Three of the five composi- 20, 1059-1062, 1992. tional phasesof the northwesternMountain Pine Ridge pluton Dawe, S.E.,The geologyof the MountainPine Ridge area and the rela- yield predepositionalLate Silurianages of 404 to 418 Ma, and tion of the MountainPine Ridge granite to the Late Paleozoicand Early the presenceof abundantsandstone, conglomerate, and red bed Mesozoicgeological history of Belize,Central America, Ms. thesis,52 p., sedimentaryfacies at the plutonmargins indicate that this igne- StateUniv. N.Y. Bitdlamton,Binghamton, 1984. Dengo,G., and Bolmenberger,O., Structuraldevelopment of northern ous body was emergentduring Santa Rosa sedimentation.The CentralAmerica, Am. Assoc.Pet. Geol.Mere., 11, 203-220, 1969. northeasternHummingbird-Mullins River pluton, thought to Dixon, C.G., Geology of SouthernBritish Honduras, With Notes on have been intrudedduring the Late Triassicbecause of abundant AdjacentAreas, 85 p., BelizeGovt. Printer, Behnopan,1956. 230 Ma K-Ar agesand marginalalterations at the plutoncon- Dmmelly,T.W., G.S. Home, R.C. Finch,E. Lopez-Ramos,Northern tacts, gave a U-Pb age of about 410-420 Ma, also predating CentralAmerica; The Maya andChortis blocks, in The Geologyof North Santa Rosa deposition.The third of the three Maya Mountains America,vol. H, The CaribbeanRegion, edited by G. Dengo,and J.E. Case,pp. 37-76, Geo. Soc.of Am., Boulder,Colo., 1990. plutons,the southeasternCockscomb pluton, •vas not dated in Dmecker,M.D., The geologyof the BladenVolcanic Series, southern this reconnaissancestudy, but coarsesedimentary facies, some Maya Mountaim,Belize, Central America, Ms. thesis,73 pp.,Colo. School possiblered-bed strata, and a singleRb-Sr age of 428+41 sug- of Mines, Golden, 1978. gest that this pluton probablyalso is older than the enclosing Flanders,R.W., Geologyof theChiqtfibul area, Belize, Central America, Pennsy!vanian-Pe__rm_iansedime•tary rocks and may have been Ms. thesis,62 pp., Univ. of Idaho,Moscow, 1978. emergentduring their deposition. Gomberg,D.N., P.O. Banks,and A.R. McBimey,Guatamala: Prelimi- naryzircon ages from central cordillera, Science, 162, 121-122, 1968. The new U-Pb agesof this studycall into questionthe widely Hall, I.H.S.,and Bateson, J.H., Late Paleozoic lavas in MayaMountains, held belief that much of the plutonic intrusionin the Maya BritishHonduras and their possibleregional significance, Am. Assoc.Pet. Mountainsoccurred in the Late Triassic.The widespreadK-Ar Geol. Bull., 56, 950-963, 1972. agesof about230 Ma are presentin the samelocalities which Harland,W.B., R.L. Armstrong,A.V. Cox, L.E. Craig,A.G. Smith,and yield Late Silurian U-Pb ages.Moreover, the Late Silurian age D.G. Snfith,AGeologtc Time Scale 1989, 263 pp.,Cambridge Univ. Press, from the northeasternpluton is derived from granite in Cambridge,1990. Kesler,S.E., J.H. Bateson,W.L. Josey,G.H. Cramer,mid W.A. Sim- immediatecontact with the contact-alteredsedimentary country mons,Mesoscopic structural homogeneity of Maya Seriesand Macal Series, rock, indicatingthat the alterationat this locality is not contact MountainPine Ridge, British Honduras,Am. Assoc.Pet. Geol. Bull., 55, metamorphism. The new ages and stone sinfilaritiesof the 97-103, 1971. contact alteration surroundingthe eastern plutons with a Kesler,S.E., C.F. Kienle,and J.H. Bateson,Tectonic sil•fificance of in- hydrothermalalteration adjacent to the marginsof the western trusiverocks in the Maya Mountains,British Honduras, Geol. Soc.Am. Bull., 85, 3307-3322. 1974. plutonsuggest that the thernmlalteration marginal to all phitons Kish,S.A., Timing of themiddle Paleozoic (Acadian) metamorphism in mightbe of hydrothermalorigin. Because the U-Pb agesindicate the southemAppalachians: K-Ar studiesin the Talladegabelt, Alabama, that intrusionin the Maya Mountainsregion did notoccur during Geology,18, 650-653, 1990. the Late Triassic, the uniform widespreadLate Triassic K-Ar Krogh,T.E., A low contaminationmethod for hydrothemialdecomposi- ages probably representthe breakup of Pangea, perhaps a tionof zirconand extraction of U andPb for isotopicage determinations, hydrothermalsignature of the rifling that formed the Gulf of Geochim.Cosmochim. Acta, 37, 485-494, 1973. Mexico. Krogh,T.E., Improvedaccuracy of U-Pbages by the creationof more concordantsystems using an air abrasiontechnique, Geochim. Cosmochim. The Late Silurianages of the Maya Mountainsplutons are the Acta, 46, 637-649, 1982. sameage as the smaller of two populationsof shockedzircons Krogh,T.E., S.L. Kamo,B. Sharpton,L. Marin, and A.R. Hildebrand, associatedwith the ChicxulubKiT boundaryimpact [Krogh et U-Pbages of singleshocked zircons linking distal K/T ejectato the Chicxu- al., 1993], suggestingthat the Maya Mountainsexposures proba- lub crater,Nature, 366, 731-734, 1993. bly are widelyrepresentative of the Yucatanbasement and of the Lopez-Ramos,E., Geologicalsummary of the YucatanPeninsula, in The basementrock impacted by the Chicxulubbolide. OceanBasins and Margins, edited by A.E.M. Nairn, and F.G. Stehli,Ple- num,New York, pp. 257-282, 1975. Ludwig,K.R., PBDAT for MS-DOS:A computerprogram for IBM-PC Acknowledgments. Initial, pilot fundingfor this studywas provided compatiblesfor processingraw Pb-U-Thisotope data, version 1.06, U.S. by NSF (EAR-9019094). Discussionswith Ron Frostand Art Snokeim- Geol.Surv. 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