Results Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol104.Issue3-4.2000:

Table 1 shows that Ar – Ar ages from horn- blende are expected to be close to, but younger than U – Pb titanite ages from the same area and older than U – Pb ages of rutile. Similarly, Ar – Ar ages of biotite are expected to be close to, but younger than, U – Pb ages obtained on rutile. This is important in the discussion of inherited or excess Ar, which appears to be very common in biotite and hornblende of granulites from the Mozambique Belt data of Priem et al. 1979 and Maboko et al. 1989, discussed below.

5. Results

Sample locations are illustrated and geochrono- logical results summarised in Fig. 2. Major and trace element analyses for most of the samples are reported by Appel 1996. Descriptions of sample locations and mineral assemblages can be found in Table 4. Several reasons — geological and analytical — may explain discordant analyses Pb-loss, over- growth core relations, incomplete dissolution. This study uses Pb-loss as the most likely interpre- tation and unless specifically stated the 207 Pb 206 Pb of discordant analyses is the most likely minimum age for this mineral fraction. Reversely discordant results are quite common with monaz- ite and often indicate excess 206 Pb from short- lived 230 Th due to preferential incorporation of Th over U during monazite growth Scha¨rer, 1984, with monazite remaining below its closure temperature. For such reversely discordant re- sults, this study uses the 207 Pb 235 U age as the best estimate for the true age since this ratio is not affected by excess 206 Pb. Because concordant re- sults could also be the result of a combination of Fig. 2. Simplified geological map with the sample locations and the U – Pb geochronological results on monazite, titanite, rutile and on one zircon fraction from the Pare and Usambara Mountains, Umba Steppe and Uluguru Moutains. Map compiled from quarter degree sheets of the Tanzanian Geological Survey. For location within Tanzania see Fig. 1. Fig. 3. Concordia diagram for monazite from the Pare and Usambara Mountains and the Umba Steppe. The results indicate an age difference of about 15 Ma. Two pairs of discordant and near concordant monazite from the Pare and Usambara Mountains are shown T115, A108. For both locations, monazite with the higher U content are more discor- dant than monazite with lower U content. 5 . 1 . Monazite and zircon Most of the monazite used in this study was separated from metasedimentary rocks. The mon- azite occur as pale to bright yellow spheres or ellipsoids free of inclusions and range in diameter from 100 to 600 mm. Two monazite fractions from the Pare Moun- tains metapelites A16 and T115b plot slightly above concordia Fig. 3. Their 207 Pb 235 U ages of 641 9 2 Ma are interpreted as the true ages of formation. Another fraction from metapelite T115 is discordant with a similar 207 Pb 206 Pb age of 640 9 2 Ma. Monazite from metapelite T137 and metagrani- toid T121 from the Usambara Mountains are also reversely discordant and yield 207 Pb 235 U ages of 621 9 2 and 624 9 2 Ma, similar to the 207 Pb 206 Pb age of 624 9 2 Ma obtained from concordant monazite fraction A108b Fig. 3. A second, dis- cordant fraction of monazite from metapelite A108a has a slightly higher 207 Pb 206 Pb age of 629 9 2 Ma. Monazite in metapelite T137 has been observed in thin section to occur mostly as large \ 300 mm grains attached to high-Ti gran- ulite facies biotite and hence as part of the high grade assemblage. Backscatter electron BSE imaging indicates strong ‘patchy’ zoning, reflect- ing zonation in Th content Fig. 4. U – Pb dating by laser-ICP-MS yielded a 206 Pb 238 U age of 618 9 15 Ma from four analyses on two grains Mo¨ller and Jackson, unpublished data support- ing the multi-grain-isotope dilution results of this study. Two other analyses indicate some Pb loss, but no evidence of older growth phases within the monazite has been found. A fraction of monazite grains from semipelite A 144 in the Umba Steppe yields a 207 Pb 206 Pb age of 609 9 2 Ma Figs. 3 and 5, the discordance 1.5 may be attributed to recent Pb-loss, possi- bly due to weathering. Four fractions of monazite were analysed from three metapelite samples of the northern, north- eastern and eastern Uluguru Mountains Fig. 6. Two monazite fractions from sample P1 show no differences in colour or grain size and differ only slightly in their Th and U contents. Fraction P1a is 1 discordant and has a 207 Pb 206 Pb age of Fig. 4. Backscatter-electron image of monazite in metapelite T137 showing pronounced patchy zoning probably related to growth inhibition at low H 2 O activity under granulite facies conditions. reverse discordance and Pb-loss, their 207 Pb 206 Pb has also to be taken as a minimum age. For age calculations of some mineral fractions rutile and some titanite very sensitive to corrections for Pb blank and initial common Pb, the 206 Pb 238 U age can be regarded as the best age estimate. 669 9 3 Ma, whereas fraction P1b is 0.6 re- versely discordant with a 207 Pb 235 U age of 657 9 2 Ma. Sample P1b could have been affected by some late disturbance, and the reverse discor- dance observed may only be the remainder of an originally much higher discordance. The age of P1a may either indicate the presence of an inher- ited Pb component detrital core or reflects the age of monazite growth during prograde meta- morphism. We consider the latter case more likely and the result of fraction P1a is, therefore, not used to calculate cooling rates for this location. However, this result may have some bearing on the discussion of the age and duration of high grade metamorphism in this part of the Uluguru Mountains. Monazite from the graphite-rich metapelite P9 has a high a 208 Pb 206 Pb ratio of 31.4 and plots significantly above concordia 2 with a 207 Pb 235 U age of 646 9 2 Ma. The monazite fraction analysed from the eastern Uluguru Mountains T28 is also slightly reversely discordant with a 207 Pb 235 U age of 653 9 2 Ma Fig. 6. The results from three metapelite samples of the northern and eastern Uluguru Mountains thus span an age range of about 11 m.y. between 646 9 2 and 657 9 2 Ma Table 2; samples P1b, P9, T28. Monazite and zircon were separated from a meta-qtz-diorite T46 originating from the north- western Uluguru Mountains. This meta-qtz-dior- ite shows evidence of all three deformational phases observed in the surrounding granulite-fa- cies rocks Appel et al., 1998. Its emplacement, therefore, must have been pre-peak-metamor- phism and pre-deformation. Two monazite frac- tions from the sample have different U – Th – Pb contents but a very similar age. The smaller mon- azite grains. with a higher ThU ratio are slightly reversely discordant at a 207 Pb 235 U age of 624 9 2 Ma. The larger monazite size fraction has a lower 208 Pb 206 Pb ratio and is slightly discordant at a 207 Pb 206 Pb age of 625 9 2 Ma. The zircon frac- tion analysed from this sample consists of 19 long-prismatic, clear and euhedral grains \ l40 m m without evidence of older cores when ob- served in transmitted light. The U – Pb result is slightly discordant 1 with a 207 Pb 206 Pb age of 626 9 3 Ma, overlapping the age of the monazite fractions Fig. 6. 5 . 2 . Titanite The geological map see Fig. 2 shows very few marbles and calcsilicate rocks in the Pare and Fig. 5. Concordia diagram for monazite, titanite and rutile from the Umba Steppe. Different fragments of the same titanite grain from sample A141 yield similar 207 Pb 206 Pb ages close to the 207 Pb 206 Pb age of 609 9 2 Ma obtained for monazite from sample A144. Fig. 6. Concordia diagram for monazite and a zircon fraction from the Uluguru Mountains. Note the age difference between monazite and zircon from the granodiorite of NW Uluguru Mountains and the monazite from metapelites of the N and E Uluguru Mountains. A . Mo ¨ller et al . Precambrian Research 104 2000 123 – 146 Table 2 U–Pb isotope data a Sample, mineral Ages in Ma Rock type Discordance f Wt. mg b Pb ppm U ppm Isotopic ratios 206 Pb 207 Pb c 208Pb206Pb d 207Pb206Pb d 206 Pb 238 U e 207Pb235U e 206 Pb 238 U 207 Pb 235 U 207 Pb 206 Pb Pare Mountains 21.50 0.082368 0.10473 0.88055 642 9 2 0.35 641 9 2 389 639 9 4 0.6 Metapelite A16, Mnz 185 597 0.8082 0.061064 0.09612 T115a, Mnz 0.80872 Metapelite 592 9 2 602 9 2 640 9 2 − 7.5 0.32 470 3050 58700 29.87 0.063380 0.10495 0.88088 643 9 2 641 9 2 2110 635 9 4 790 1.3 280 T115b, Mnz Metapelite 0.15 610 Calcsilicate 0.6612 0.083598 0.09663 0.80160 595 9 2 598 9 2 610 9 3 − 2.5 5.49 19.4 130 A26, Tit 167.4 Metabasite 0.9988 0.146452 0.08937 0.74589 552 9 10 566 9 9 623 9 16 − 11.4 4.52 18.2 98 T114, Tit 0.0181 0.061373 0.08635 0.69205 534 9 2 534 9 2 2140 535 9 4 A16, Rt − 0.1 6.86 0.55 13.31 Metapelite 0.2226 0.058517 0.08517 0.68126 527 9 2 528 9 2 T115, Rt 530 9 4 Metapelite − 0.6 10.89 2.9 31.1 9750 Usambara Mountains 23.75 0.064764 0.10173 0.84275 625 9 2 0.40 621 9 2 1260 606 9 2 3.0 570 Metapelite T137, Mnz 2640 4210 28900 3.189 0.061022 0.10041 0.84068 617 9 3 620 9 3 629 9 2 − 2.0 0.22 A108a, Mnz 1560 Metapelite 3020 22600 3.382 0.060666 0.10182 0.85021 625 9 2 625 9 1 624 9 2 0.2 0.12 A108b, Mnz 1180 Metapelite 100.3 0.061912 0.10178 0.84821 625 9 2 624 9 2 4470 619 9 2 T121, Mnz 0.9 480 4260 0.22 Charnockite 0.3119 0.178682 0.08191 0.64797 508 9 2 507 9 4 T137, Rt 506 9 17 Metapelite 0.3 4.00 4.89 39.5 118.3 0.0446 0.070951 0.08384 0.66902 519 9 2 520 9 2 830 525 9 4 T139, Rt − 1.1 8.2 0.67 8.30 Meta-qtz-di. A108, Rt 0.0718 Metapelite 0.059622 0.08526 0.68112 527 9 2 527 9 1 527 9 3 0.0 12.78 1.14 13.7 4050 Umba Steppe 1170 9730 5.949 0.061302 0.09740 0.80778 599 9 2 601 9 2 609 9 2 − 1.6 0.50 A144, Mnz 690 Grt-Bt gneiss 0.0181 0.061365 0.07358 0.61103 458 9 2 484 9 2 12300 612 9 2 A141a, Tit − 25.2 2540 174 2.82 Marble 0.0502 0.061741 0.09333 0.77705 575 9 2 584 9 2 A141b, Tit 617 9 2 Marble − 6.8 2.38 111 1240 7060 0.0371 0.071373 0.08306 0.65759 514 9 2 513 9 2 928 508 9 7 A144a, Rt 1.3 36 2.9 3.70 Grt-Bt gneiss A144b, Rt 0.0025 Grt-Bt gneiss 0.057805 0.08318 0.66102 515 9 5 515 9 4 516 9 3 − 0.2 15.52 2.4 32 16800 Uluguru Mountains 0.0810 0.062140 0.10075 0.84223 619 9 2 T46, Zrn g 620 9 2 Meta-qtz-di. 626 9 3 − 1.2 0.59 37.1 370 6100 18.52 0.064354 0.10180 0.84866 625 9 2 624 9 2 2140 620 9 3 Diorite 0.7 T46a, Mnz sm 770 1340 0.08 8300 Diorite 11.88 0.061860 0.10106 0.84457 621 9 2 622 9 2 625 9 2 − 0.8 0.18 2460 2170 T46b, Mnz la 1040 6110 7.995 0.063559 0.10828 0.92330 663 9 2 664 9 2 669 9 3 − 0.9 0.25 P1a, Mnz 880 Metapelite 8.514 0.063433 0.10740 0.90936 658 9 2 657 9 2 5060 654 9 2 910 0.6 1020 P1b, Mnz Metapelite 0.20 2190 Metapelite 31.37 0.065035 0.10612 0.88956 650 9 2 646 9 2 632 9 3 2.9 0.23 960 320 P9, Mnz 7370 Metapelite 4.621 0.062217 0.10683 0.90205 654 9 2 653 9 2 648 9 2 1.0 0.19 510 980 T28, Mnz 0.9208 0.092502 0.09415 0.78521 580 9 6 588 9 7 453 621 9 22 P8a, Tit − 6.6 260 43.6 4.71 Calcsilicate 280 553 0.7989 0.086545 0.10183 0.84817 625 9 11 624 9 8 618 9 4 1.1 P8b, Tit Calcsilicate 2.79 48.1 0.4862 0.081095 0.09893 0.82422 608 9 5 610 9 5 681 619 9 7 Marble − 1.7 T25a, Tit 115 15.8 5.78 630 Marble 0.0504 0.082931 0.09638 0.80358 593 9 4 599 9 3 621 9 4 − 4.4 4.76 10.8 112 T25b, Tit 831 Marble 0.5163 0.077363 0.09989 0.83277 614 9 7 615 9 5 620 9 3 − 1.0 4.74 17 121 T25c, Tit 0.0479 0.105575 0.09956 0.82986 612 9 6 614 9 5 317.5 620 9 6 P88a, Tit − 1.3 116 12.2 4.43 Calcsilicate 0.4831 0.100277 0.09992 P88b, Tit 0.83243 Calcsilicate 614 9 3 615 9 2 621 9 5 − 1.1 3.83 19.5 136 359.1 0.0314 0.063062 0.08853 0.71904 547 9 2 550 9 1 2740 564 9 2 2.1 − 3.0 25.3 P1, Rt Metapelite 10.27 1910 Metapelite 0.2263 0.064848 0.08909 0.71589 550 9 4 548 9 3 540 9 3 1.8 12.02 2.5 25.5 P9, Rt 25.3 2120 0.0976 0.062982 0.08065 0.63767 500 9 5 501 9 4 505 9 5 − 0.9 T28, Rt 12.2 Metapelite 2.0 a Ages and errors 2s are calculated with Pbdat and ISOPLOT for Excel 2.0.6 software, after Ludwig Ludwig, 1980, 1994; Zrn, zircon; Mnz, monazite; Tit, titanite; Rt, rutile; sm, small; la, large. b Most Mnz weights estimated from size, error about 10–20, other samples weighed toB1 error. c Measured ratio. d Measured ratio, corrected for spike, 80 pg Pb blank, 0. 1 mass fractionation per a.m.u. e Corrected for spike, 80 pg Pb blank, 0. 1 mass fractionation per a.m.u. and common Pb composition determined from leached coexisting K-feldspar or plagioclase Mo¨ller et al., 1998. f Discordance of result expressed as deviation of the 207 Pb 206 Pb age from the 206 Pb 238 U age = [ 207 Pb 206 Pb age 206 Pb 238 U age100]−100. g Ninteen clear, pink, euhedral, prismatic grains, \140 mm mesh size, length to width ratio \6. Fig. 7. Concordia diagrams for titanite. isotope composition of A26 as no analysis of coexisting plagioclase was available. The possible error associated with the correction is small con- sidering the narrow range of common Pb compo- sition of feldspars from the Pare and Usambara Mountains Mo¨ller et al., 1998. The result sug- gests that cooling through the closure temperature of titanite in the South Pare Mountains occurred in the same age range as in the North Pare Mountains. In the Umba Steppe interlayered calcsilicate rocks and marbles were found at the Umba river. A coarse-grained sample A141 yielded an opaque to very dark brown titanite grain of more than 0.5 cm diameter. Two fragments from the core of the grain have variable U and Pb concen- trations and degree of discordance Table 2, Fig. 5 but similar 207 Pb 206 Pb ages of 612 9 2 and 617 9 2 Ma. The high U content of the titanite fragments may have caused structural damage and Pb-loss. It can be concluded that the 207 Pb 206 Pb ages record the age of peak metamorphism and the effective closure temperature of this titan- ite grain is higher than the ca. 730°C calculated for grains with 0.5 cm diffusion radius by Cher- niak 1993 or alternatively that metamorphic temperatures did not exceed T c after titanite growth. Suitable titanite-bearing samples were only found in the eastern part of the Uluguru Moun- tains P8 and T25 close to the locations of the monazite and rutile samples. An additional sam- ple was taken from the southeast Uluguru Moun- tains P88. To evaluate the reproducibility of U – Pb ages of titanite from the eastern Uluguru Mountains, several fractions of grains were analysed for each sample. Their 207 Pb 206 Pb ages overlap within error at 618 – 621 Ma. The variably discordant titanite fractions can be fitted on a single regression line Fig. 7b despite being taken form localities about 60 km apart. The combined intercept of titanite in the eastern Uluguru Moun- tains at 619 9 2 Ma is interpreted as the age at which at least this part of the Uluguru Mountains the crystalline limestone group of Sampson and Wright 1964 cooled through the closure temper- ature of titanite at ca. 650°C. Usambara Mountains. Only two suitable samples of titanite-bearing rocks could be collected A26, T114 from the Pare Mountains. Sample A26 from the North Pare Mountains is a calcsilicate gneiss with the metamorphic assemblage Grt+ Cpx+Hbl+Pl+Qtz+Cc+Scp+Tit9Kfs. The titanite is reddish-brown and does not exhibit colour zonation. The 207 Pb 206 Pb age of 610 9 3 Ma Fig. 7a is interpreted as the minimum age for closure at ca. 650°C. Titanite in metabasite T114 from the South Pare Mountains is pale brown and only 150 to 200 mm in diameter. It has a low 206 Pb 204 Pb ratio of 167.4 and yields an imprecise 207 Pb 206 Pb age of 623 9 16 Ma. Com- mon Pb correction was carried out with the Pb 5 . 3 . Rutile The rutile fractions used for U – Pb age determi- nations were obtained from metapelitic samples that also yielded monazite A16, T115, T137, A108, A144, P1, P9, T28. Rutile in the metapelitic samples were mostly elongate grains aspect ratio higher than 4 or fragments thereof. In all of these samples rutile is part of the high- pressure granulite-facies assemblage together with garnet, sillimanitekyanite, plagioclase, quartz 9 ilmenite. An exception is qtz-dioritic enderbite T139 from the western Usambara Mountains which contains large, dark, short rutile grains with an average diameter \ 250 mm. Rutile from a single sample often spans a range of colours from translucent reddish brown to almost opaque and dark-brown to black. Care was taken to pick grains of similar size and colour for each rutile fraction 4 – 16 mg to avoid mixing of different chemical compositions and possibly different dif- fusion behaviours. Concordant rutile fractions yield 207 Pb 206 Pb ages of 535 9 4 and 530 9 4 Ma for the North and South Pare Mountains, respectively Fig. 8a. Re- sults of two rutile fractions from meta-qtz-diorite T139 and metapelite A108 from the Usambara Mountains have indistinguishable 207 Pb 206 Pb ages of 525 9 4 and 527 9 3 Ma, respectively but sig- nificantly different 206 Pb 238 U ages of 5 19 9 2 and 527 9 2 Ma. Rutile from sample T137 is still younger with a 206 Pb 238 U age of 508 9 2 Ma, but it is slightly discordant and has a large uncer- tainty in 207 Pb 206 Pb 506 9 17 Ma due to its low proportion of radiogenic Pb. Sample T139 was collected just 10 km from metapelite T137 and belongs to a suite of meta-qtz-diorites, some of which cross-cut the foliation of the surrounding granulites. Its rutile age is 11 Ma older than that of rutile from sample T137 and could possibly be explained by the larger diffusion radius of the stubby rutile grains from the meta-qtz-diorite. Two rutile fractions from the Umba Steppe were picked from the same sample of semipelitic gneiss as the monazite A144 and yield similar 206 Pb 238 U ages of 515 9 5 and 514 9 2 Ma Fig. 5. The two rutile fractions from metapelite sam- ples P1 and P9 of the northern Uluguru Moun- tains are normally and reversely discordant, respectively. There is no reason to assume that rutile could be affected by excess 206 Pb since rutile generally has extremely low Th contents. Unless other geological problems are responsible for the discordance, the best estimate of the ‘true’ age is probably the 206 Pb 238 U age see discussion above. The 206 Pb 238 U ages are indistinguishable within error at 550 9 4 and 547 9 2 Ma. They are interpreted to date the time the rocks cooled below T c . Rutile from metapelite T28 is concor- dant and has a 206 Pb 238 U age of 500 9 5 Ma Fig. 8b. The slightly discordant ages at ca. 550 Ma from the northern Uluguru Mountains are also about 15 Ma older than the oldest rutile age Fig. 8. Concordia diagrams for rutile. obtained in the Pare and Usambara Mountains, a similar age difference as observed with the monazite.

6. Discussion