JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B8, PAGES 17,727-17,735, AUGUST 10, 1996 Late Silurian 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 Permian 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 Triassic,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 Pennsylvanian 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 ß i'i:..:"." . .i.. ß'.'..:....' ß Ocean ß ß ß ß o o Pacific o". Ocean Hummingbird Batholith Mountain Pine Ridge Batholith ß U-PbSample Cockscomb Batholith :..• Post-CretaceousSediments .....• CretaceousLimestone ß . ..... ß .ß .. ß . ... ß . ... ß"'. '..'."' •;• SantaRosa ß....... : . .. Group ..... -,-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 so uniform;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-Ordovician 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