Canadian Mineralogist Vol. 29, pp. 319-336(1991) NEPHELINE.RICHFOIDOLITES AND RARE.EARTHMINERALIZATION IN THE EL PICACHOTERTIARV INTRUSIVE COMPLEX, SIERRADE TAMAULIPAS, NORTHEASTERNMEXICO MARIANO ELIAS.HERRERA,RAUL RUBINOVICH-KOGAN, RUFINOLOZANO-SANTA CRUZ ANDJOSE LUIS SANCHEZ.ZAYALA Instituto de Geologfa, Universidad Nocional Autdnoma de Mdxico, Ciudad Universitaria, Delegacidn Coyoacdn, M510 Mdxico, D.F., Mdxico ABSTRACT suresriches en apatite et enrichiesen terresrares. Cesfoido- lites contiennent n6phdline (55-9190), pyroxEne (entre The El PicachoTertiary intrusivecomplex is locatedin Di36Hd3eAe25et Dit8Hd3zAe45)(4-3590), et orthoclase the west-centralpart of the Sierra de Tamaulipas,in the (0-220/o).Ce sont desroches sous-satur6es en silice(44-50q0 alkaline province of northeastMexico. The complexcuts de SiO2 en poids) qui sont fortement hyperalcalines Cretaceouslimestone, and it consistsof gabbro, kaersu- (11-1590 de Na2O + K2O) et d faibles teneus en Mg tite diorite, syenite, nepheline-richfoidolites (ijolite, urtite, (l-490 MgO), et qui correspondraiente destermes 6volu6s juvite), and trachytic and phonolitic dikes.The nepheline- d'une s€rie fractionnde d partir d'un parent aussi sous- rich foidolites are affected by alkali metasomatismand con- satur6. La metasomatosea donnd des f6nites d aegyrine tain apatite-rich veins enriched in the rare-earth elements. l6gdrement hyperalcalines, qui contiennent albite, The nepheline-richfoidolites consistof nepheline(55-91 9o), orthoclase, cancrinite en pseudomorphosede la n€ph61ine, pyroxene (4-35V0)(Di36Hd3eAe25 to Dil8Hd37Ae45)and et un pyroxbne sodique (entre Di14Hd21Ae5set' orthoclase(0-22V0), They are silica-undersaturatedrocks DilHd27Ae7r. Nous trouvons aussides fdnites leucocrates, (44-50 wt.olo SiOt with a strongly alkaline character i onhoclase + albite, avecde faiblesquantites de quartz, (11-15wt.9o Na2O + K2O)and low contentsof Mg (l-4 interstitiel. Les fissuresriches en apatite contiennentdes w.q0 MgO), correspondingto fractionatedproducts of a agr6gatsfibroradi€s d'apatite, chalc6doihesph6rolitique, silica-undersaturatedparental magma. The alkaline quartz, sid€rite, calcite, barytine et britholite, qui est le por- metasomatismis representedby a slightly peralkaline aegi- teur principal desterres rares. Les fissuresont une teneur rine fenite, which consistsof albite, orthoclase,cancrinite €lev6een terresrares, entre 14,6A0et29,600 ppm, et mon- pseudomorphs after nepheline, and Na-pyroxene trent un fort enrichissementen terresrares l6gbres (308 < (DilaHd27Ae5eto Die1Hd27Ae72),Mafic-mineral-free fenite La/Ltt < 2258).Une immiscibilitd entre liquides silicat€ alsois found in the complexand consistsof orthoclaseand et carbonat6serait i I'origine desliens p6trogdndtiquesentre albite, with minor amounts of interstitial quartz. The les foidolites d n€ph6line,la metasomatosequi a produit apatite-rich veins are composedof radial aggregatesof apa- les fdnites,et les fissuresi apatiteenrichies en terresrtues tite, spherulitic chalcedony,quartz, siderite, calcite, barite, l6gdres. and britholite, which is the main R.EE-mineralin the veins. The total REE content in the veins varies from 14600to (Traduit par la R6daction) 296@ppm, with strong enrichmentin ZREE (La,/Lu ratios rangefrom 308to2258). A silicate-carbonateliquid immis- Mots-clds:foidolites, rochesir feldspathoides,min6ralisa- cibility model is postulatedand discussedin order to explain tion en terresrares. rochesalcalines, britholite, com- coherentlythe petrogeneticrelationship of the nepheline- plexescarbonatitiques, Tamaulipas, Mexique. rich foidolites with fenite metasomatismand apatite-rich veins enriched in LREE. INTRoDUcTIoN Keywords: foidolites, feldspathoidal rocks, REE minerali- zation, alkaline rocks, britlolite, carbonatitecomplexes, The northeastern part of Mexico, from the Gulf Tamaulipas,Mexico. Coastal Plain near Tampico to the U.S.A. border at the Rio Grande (Fig. l), is characterized by the SoMMAIRE presence of several Tertiary alkaline intrusive com- plexes. They cut Cretaceous limestones, which are Le complexeintrusif tertiaire de El Picacho est situd dans gently folded by the Laramide Orogeny. The mag- le secteurouest-central de la Sierrade Tamaulipas,et fait matic suites form part of the alkaline province of partie de la provincealcaline du nord-estdu Mexique. Le eastern Mexico (Demant & Robin 1975, Cantagrel complexerecoupe un calcairecrdtac€, et contient desunitds Robin 1982), which continues toward de gabbro, diorite kaersutitique,sy6nite, foidolites i ndph6- & Robin 1979, province line (ijolite, urtite, juvite), ainsi que desfilons de trachyte the U.S.A. in the Trans-Pecos of Texas. et de phonolite. Les foidolites richo en n6ph6linesont affec- The presence of alkaline suites in northeastern t€espar une m6tasomatosealcaline, et contiennentdes fis- Mexico has not been well documented in petro. 319 320 THE CANADIAN MINERALOGIST I rO Iooo RA NDE RIFT Ifl$rU.S. A. NTRANS-PECOS , \PROVI NCE Yo \ \ffi ! MEXICO p N \rr \ 26o ..\1, M'NTERREYs li /\ I \ 250 Km \...:i fh \t)r\t.o u,,,0,,0 \ rE PICACHO EL __. _ 6 INTRUSIVE COMP.LEX o \) Foulls / -n"o,".,'Arvrrr\,u l04o IOO'o FIc. 1 Location map of the El Picacho intrusive complex. The normal faults are after King (1969). graphic, geochemicaland geochronologicalterms; to havebeen associated with a possibleextension of accordingly, their tectonic significanceis poorly con- the Rio Grande rift toward the southeast(Bloom- strained.For example,few K-Ar agesrelated to alka- field & Cepeda-Ddvila1973, Robin 1982).However, line magmatismat Tamaulipashave been reported: the Trans-Pecosmagmatism, which apparentlyhas 29.6 + l.2Mato 27.9 t 1.1Ma for syeniticrocks an obvious geographicrelation to the Rio Grande from San Carlos alkaline complex (Bloomfield & rift (Fig. l), is consideredto be the far-inlandexpres- Cepeda-D{vila1973) located 80 km north of Ciudad sion of the volcanismproduced by the subduction Victoria, 28.0 t 0.8 and 23.5 + 0.7 Ma for a of the Farallon plate along the Pacific coastduring nephelinite volcanic neck and nepheline syenite the Tertiary (Barker 1979, 1987,Price et al. 1987). dome, respectively(Cantagrel & Robin 1979),both This argumentis supportedby the observedcomposi- hypabyssalalkaline rocks located in the CoastalPlain tional gradation of the Trans-Pecosalkaline mag- nearTampico, and 7.0 + 0.2 Ma for phonolitedikes matism toward the southwestto calc-alkalicmag- in the central part of the Sierra de Tamaulipas (Can- matism of the Sierra Madre Occidentalin western tagrel & Robin 1979). Mexico. The alkalinemagmatism is consideredby someto The El Picacho intrusive complex, of alkaline be typical of an intracontinentalrift zone, without affinity and located approximatly 65 km southeast relation to the compressiveregime of the Pacific sub- of Ciudad Victoria in the west-centralpart of the duction (Cantagrel& Robin 1979,Robin 1982),and Sierra de Tamaulipasin northeasternMexico (Fig. l$Fi I-.],'la p,,:)i,t"-Y, t', I t , ) , -'.r','/ x i*,+++++++++++++i +--iEiT, +. +.* x |I i f f 't + + r x x r 1 r x;+.++.+; + +++ J+ ++.+ + + +:+.+l I +r +i+f +f++ + r +l I t x r x x xl +,++ +,+ ++. +.+'i txxxxxrxxrxxt l+ r+ r+ r+r +r +r+ I t Xt X XI Xt XI XI | .+.+.+.+.+.r + +.+4 + xxxtxx)(xt(xxlx l f +'+ + i- 1- + W Trochyteond phonotite dikes Nl creloceouslimeslone ft lT:{{r1+lt r tr Nepheline- rich f oidolif es I | (ijotite,urtile, juvite) 1 I F Syenile House E Fsitl ul APTR.9 Koersutitediorile . F |Tttl Sompl€ locoiion r rTl ilE-e53 F Gobbro I r r rl ---- Rood Ftc. 2. Simplified geologicalmap of El Picachointrusive complex and a cross-sectionthrough the complex.Inset: area shownin detail in Fig, 5. 322 THE CANADIAN MINERALOGIST l), has never beenrecognized in the past, and was sutitediorite. It is observedonly in the northwestern describedas a Tertiary gabbroic intrusive body in part of the El Picachoarea (Fig. 2). The kaersutite unpublishedtechnical reports only. In order to con- diorite is a medium-grainedmesocratic rock consist- tribute to a better knowledge of this alkaline ing of plagioclase(An36r), kaersutite,and accesso- province,the presentpaper is devotedto a descrip- ries suchas clinopyroxene,olivine, titaniferousbio- tion of the El Picacho intrusive complex, with tite, titaniferous magnetite,ilmenite and apatite. The emphasison somepeculiar characteristicsthat make diorite haslocal variationsto bandedmonzodiorite it a unique exampleknown so far in Mexico: presence in which the thin layers(0.5-3 cm) rich in kaersutite of nepheline-richfoidolites, fenites,and apatite-rich alternatewith leucocraticlayers rich in plagioclase veinswith high contentsof the rare-earthelements and K-feldspar. (REE),The complexis exposedin a small ellipsoi- dal area of about 9 kmz (Fie. 2). Its age is Syenite unknown, but by extensionof the K-Ar data men- tioned above,it probably wasemplaced in the inter- Betweenthe gabbro-diorite and the Cretaceous val Oligoceneto Pliocene. limestone,bodies of leucocraticsyenite are found. In someplaces (ME49 , M8256, Fig. 2), the syenite PETRoGRAPHY can be seento cross-cutthe gabbro. The contact between the syenite and the limestone is Gabbro predominantly subhorizontal and parallel to the stratification, and suggeststhat the syenitebodies The gabbro is the most abundantigneous unit in are mainly tabular, as is depictedin the cross-section the El Picachoarea (Fig. 2), and the oldestrock in through the complex (Fig. 2). the igneouscomplex, according to cross-cuttingrela- The leucocratic syenite is a fine- to medium- tionships. It is a medium-