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Partial Melting of Fenitized Crustal Xenoliths in the Oldoinyo Lengai Carbonatitic Volcano., T Rnzania: Reply

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Partial Melting of Fenitized Crustal Xenoliths in the Oldoinyo Lengai Carbonatitic Volcano., T Rnzania: Reply A merican M i neralo gist, Volume73,pages 1468-1471. 1988 Partial melting of fenitized crustal xenoliths in the Oldoinyo Lengai carbonatitic volcano., T rnzania: Reply Rosrnr F. MlnrrN Department of Geological Sciences,McGill University, 3450 University Street,Montreal, Quebec}{3A 247, Canada Vronrc.q.Monoc.qs GeologiskaInstitutionen, Stockholms Universitet, Box 6801,S-113 86 Stockholm,Sweden Ansrn-q,cr There is perhaps no more exotic an igrreousrock type than natrocarbonatite, erupted from the volcano Oldoinyo Lengai, in northern Tanzania. Its origin is still the object of debate. Gittins (1988) has proposed that the alkalic carbonatite magma is a relatively evolved derivative of a mantle-derived olivine-bearing sdvite. We agree that it is geo- chemically an evolved magma, as are the associatedsilicate lavas; however, the bulk of the evidencesuggests to us that the natrocarbonatitic magma has a crustal component and may well have formed within the crust. In this essay,we review the backgroundinformation pertinent to this proposal, made in our paper on xenoliths of fenitized crustal material brought up in the natrocarbonatite lava (Morogan and Martin, 1985). INrnooucrrox "metasomatic facies," charucrerizedby high-grade, me- The investigation of Morogan and Martin (1985) was dium-grade, and low-grade fenites, could be measuredin motivated by a natural curiosity concerning the assem- centimetersor meters,as might be expectednear the out- blage of xenolithic materials of crustal origin brought to er, near-vertical contact of an intrusive alkaline complex the Earth's surfacein the highly unusual natrocarbonatite emplaced in cold host-rocks near the Earth's surface,or lava ofOldoinyo Lengai volcano, located in the southern on either side of a crack along which the fenitizing fluids part of the Gregory Rift, in northern Tanzania. The focus are percolating.But the metasomaticinteraction was con- of our investigation was a suite of nine hand specimens sidered to be developed on a broader scaleby Morogan offenitized rocks, and not the host natrocarbonatitelava. and Martin (1985), in part becauseof the anomalous We realized at the outset that as in any investigation of thermal gradient found in this area (cf. the "alkaline a suite of metasomatic rocks in which the unmetasoma- province geotherm"of Joneset al., 1983). tized starting material is unexposedand unsampled,ours "That fenitization might occur on a regional scale . was perforce an open:ended survey of the phase assem- is a new idea," saysGittins (1988, p. 1465).We were blagesdeveloped prior to and during entrainment in the attracted to it becauseofthe unusual regional geophysical unusual lava. Clearly, therefore, we did not set out to characteristicsof the upper mantle and lower crust along addressthe evidently controversial question of the ulti- various parts of the East African Rift. An elliptical area mate origin of the Oldoinyo Lengai natrocarbonatite. In (the Kenya dome) approximately 900 km along its short our Discussion section, however, and specifically in the axis dominates the easternrim of the East African pla- last two paragraphsofthe paper, we did proposea petro- teau; the dome results from a maximum uplift of 1.5 to geneticscheme that could explain, at leastin a qualitative 2 km since mid-Tertiary time (Baker and Wohlenberg, way, the variety ofigneous products in this sector ofthe l97l; Savageand Long, 1985).The GregoryRift, a 70- rift system,as describedby others, and the metasomatic km-wide zone of normal faults, bisectsthe domal feature phenomena that we had just documented. We welcome along its crest.Regional gravity and seismicsurveys show this unexpected opportunity to clarify our views and that the dome reflects the presenceof a broad elliptical evaluatethe backgroundinformation that led to our nov- area of anomalous upper mantle. The mantle below the el proposal that the natrocarbonatite could have origi- Gregory Rift has an anomalously low specificgravity of nated in the suite of metasomatizedcrustal rocks rather 3.2 and is characterizedby attenuatedP- and S-waveve- than deep in the upper mantle. locities. At the normal depth of the Moho, a 30-km-wide bulge of anomalous upper mantle crossesinto the lower Sclr,e oF THE METASOMATTCTRANSFoRMATIoNS crust, and narrows upward (Savageand Long, I 985). This There is obviously nothing recorded in the nine hand lower crust, characterizedby a P-wave velocity of 7.3 specimensof fenitized crustal rocks to justify a firm state- km/s, presumably represents a mixture of shield-type ment one way or the other concerning the scale of the lower-continental crust, as characterizedoff the rift axis zones of metasomatic assemblages.The width of the (6.5 km/s), and alkaline igneousbasic and ultrabasic ma- 0003-004x/88/I l l 2-1468$02.00 1468 MARTIN AND MOROGAN: ORIGIN OF ALKALIC CARBONATITE LAVA-REPLY r469 terial crystallized from magmas injected from the upper into the crust, which is in a state of tension becauseof mantle (Maguire and Long, 1976; Mooney et al., 1983; regionaldoming, or very efficiently,as direct "emanations Savageand Long, 1985).Mooney et al. (1983)considered from below." There remain problems concerning the the development of a lower crust modified in this way to mechanismsof migration of a discrete volatile phase in be a common feature of continental rifts. The proportion the mantle, but the evidence for regional metasomatism of intruded material probably decreasesupward; the rel- in the mantle beneath continental rifts, in our opinion, is atively high density inferred for the upper halfofthe crust compelling. (3.05 g/cm3)is consistentwith the presenceof igneous ..EMANATIoNS bodies there as well and with a concomitant alteration of N,trunn oF THE FRoM BELowtt the crust. The area is consideredgenerally to be in iso- Recent contributions on the topic of mantle metaso- static equilibrium, and domal uplift is viewed as the re- matism make a clear casefor the introduction of a suite sponseto the presenceof a regional low-density mass in of incompatible elements, including the alkalis, but are the upper mantle. less categorical concerning the respective roles of HrO Heat lossalong the GregoryRift (l l-30 MWkm: Crane and COr, which are likely to be the two dominant species and O'Connell, 1983) is focusedthrough the central part in the relatively oxidized continental lithosphere (Eggler, of the Kenya dome, but geothermal manifestations do 1987). The two speciesare likely to play diferent roles, occur on a regional scale.The heat flow is consideredan HrO effective in complexing most major and trace ele- expressionof focusedmantle degassing(Bailey, 1983)and ments in the solute, and CO' more effectivein mobilizing the unusual intensity of injections of mantle-derived the alkalis and the rare earths.The relevant experimental magmas into the crust. In our opinion, fenitization of dat'aare still sparse.In the opinion of Bailey ( I 980, I 983), crustal rocks could be effectedby both agents,i.e., (l) a the important volatile constituents to escapefrom the fluid medium releasedlocally along an intrusive contact mantle into continental rift systemsare CO2,HrO, F, and as a result of crystallization of an alkaline magma derived cl. from the anomalous mantle below and (2) a fluid phase Joneset at. (1983) have documentedmetasomatic re- releasedfrom a leaky anomalous mantle. Is the shortest actions involving CO, in the lower crust in the Gregory distancebetween two intrusive contactsin this zone mea- Rift: relicts of olivine-normative alkali gabbrosare meta- sured in terms of meters or in kilometers? We believe morphosedinto mafic granulitesthat contain sulfate- and that there is such an intense veining of the crust that a carbonate-rich meionitic scapolite.These xenoliths were proposal of a regionally affected,metasomatically modi- collected from the Lashaine tuff cone in northern Tan- fied crust below the Oldoinyo Lengai area is not to be zania.The development of scapoliteis consideredmainly consideredfar-fetched. a result of CO, metasomatism at an unusually high re- What is going on geochemicallyin the anomalous up- gionally developedtemperature (1200 K) at a pressureof per mantle under the Kenya dome obviously is relevant 14 kbar, but HrO is likely also involved, as in the recon- to the question ofthe nature ofthe fluid leaking into the structive transformation of any silicate (Donnay et al., crust above. As in the caseofour knowledgeconcerning 1959).Metasomatism involving K (and HrO?) seemsto the crust, any insight into the state of the upper mantle have occurred later, near the surface,after the incipient is based on meager samples brought to the surface in decompression-relatedmelting recordedin the xenoliths. eruptions of alkali basalt and nephelinite and on geo- In a companion study, an important 2.0 Ga event of physical modeling. Textural and mineralogical studies of chemical fractionation involving the U-Pb, Sm-Nd, and ultramafic xenoliths from this and other areas along the Rb-Sr systemswas discovered in garnet lherzolite xen- East African Rift have revealed abundant evidence for oliths from Lashaine by Cohen et al. (1984). The pref- regionalmetasomatic enrichment ofthe upper mantle via erential mobilization of U, Pb, Rb, and Sr probably re- a mobile fluid, considereddominantly COr-bearing(Lloyd flects the influence of HrO more than COr. and Bailey,1975; Bailey, 1980, 1982; Cohen et a1.,1984; Morogan and Martin (1985) did not investigatethe na- Dawson, 1987;Lloyd, 1987).There is clearevidence for ture of trapped
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