Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

Rojas, Gaston Eduardo Enrich; Ruberti, Excelso; Azzone, Rogério Guitarrari; Gomes, Celso de Barros

doi:10.1590/2317-4889201620160075

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Brazilian Journal of Geology

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ARTICLES • Braz. J. Geol. 46 (03) • Jul-Sep 2016 • https://doi.org/10.1590/2317-4889201620160075 copy

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

Minerais do grupo da eudialita da suíte alcalina Monte de Trigo, Brasil: composição e implicações petrológicas

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT:

The Monte de Trigo alkaline suite is a SiO₂-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs) occur in one nepheline microsyenite dyke, associated with aegirine-augite, wöhlerite, låvenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu ), Nb (0.31- 0.76 apfu ), Fe (1.40-2.13 apfu ), and Mn (1.36- 2.08 apfu ). The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu ), moderate enrichment of rare earth elements (0.38-0.67 apfu ), and a relatively low Na content (11.02-12.28 apfu ), which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce), georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm), Mg (1,179-3,582 ppm), and Zn (732- 852 ppm) than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange.

Keywords:
mineral chemistry; agpaitic rocks; Serra do Mar alkaline province

Analysis^a	1	2	3	4	5	6	7	8	9
wt%^b					5	6	7	8	9
SiO₂	47.62	46.60	47.12	47.26	46.02	46.86	46.75	46.83	47.48
Al₂O₃	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	0.02	<0.01	<0.01
TiO₂	0.33	0.22	0.25	0.34	0.16	0.23	0.23	0.21	0.23
ZrO₂	11.94	11.78	11.85	11.98	11.57	11.78	11.77	11.84	11.78
HfO₂	0.23	0.22	0.25	0.22	0.25	0.21	0.23	0.26	0.24
Nb₂O₅	2.05	2.52	2.46	1.91	3.09	2.57	2.71	2.68	2.35
Ta₂O₅	0.20	0.23	0.19	0.17	0.22	0.20	0.16	0.25	0.25
Y₂O₃	0.86	0.79	0.86	0.86	0.73	0.80	0.82	0.80	0.85
REE₂O₃	2.93	3.36	3.13	2.38	3.43	3.09	3.00	2.86	2.72
FeO	3.33	3.17	3.24	3.44	3.28	3.56	3.51	3.13	3.28
MnO	4.00	4.34	4.23	3.88	4.52	4.05	4.20	4.30	4.09
CaO	12.51	12.01	11.96	12.04	12.01	12.04	11.84	12.27	12.12
Na₂O	10.73	11.37	11.50	11.86	11.33	11.45	11.48	11.29	11.46
K₂O	0.24	0.23	0.25	0.28	0.27	0.33	0.23	0.23	0.24
Cl	1.75	1.73	1.73	1.78	1.67	1.68	1.74	1.75	1.77
F	0.20	0.25	0.23	0.27	0.17	0.14	0.18	0.24	0.31
c
H₂O	1.75	1.66	1.71	1.65	1.75	1.81	1.74	1.68	1.63
-O=F,Cl	0.48	0.50	0.49	0.51	0.45	0.44	0.47	0.50	0.53
Total^d	100.43	100.27	100.75	99.97	100.35	100.67	100.39	100.39	100.51
Structural formulae (a.p.f.u.)^e					100.35	100.67	100.39	100.39	100.51
Si	25.226	25.126	25.150	25.227	25.024	25.133	25.106	25.096	25.218
Zr	3.085	3.097	3.084	3.119	3.066	3.080	3.081	3.094	3.051
Ti	0.129	0.087	0.101	0.135	0.064	0.092	0.094	0.083	0.093
Nb	0.490	0.615	0.593	0.460	0.759	0.622	0.657	0.650	0.565
Fe	1.476	1.429	1.446	1.534	1.491	1.596	1.577	1.402	1.455
Mn	1.794	1.981	1.913	1.756	2.083	1.838	1.912	1.951	1.839
REE	0.559	0.652	0.600	0.471	0.673	0.597	0.580	0.569	0.536
Ca	7.098	6.937	6.839	6.886	6.996	6.916	6.811	7.046	6.896
Y	0.242	0.226	0.244	0.243	0.211	0.228	0.234	0.228	0.241
Na	11.016	11.890	11.899	12.277	11.939	11.910	11.951	11.724	11.802
K	0.163	0.159	0.172	0.189	0.184	0.228	0.155	0.156	0.163
F	0.340	0.428	0.390	0.447	0.296	0.232	0.299	0.402	0.521
Cl	1.571	1.583	1.568	1.613	1.539	1.531	1.587	1.591	1.594
H	3.089	2.989	3.042	2.940	3.165	3.237	3.114	3.007	2.885
Trace elements (ppm)^f					3.165	3.237	3.114	3.007	2.885
Mg	103	248	146	72	154	181	150	157
Sc	25	27	25		29	29	26
Zn	467	535	477		495	505	401
Ga	73	97	91		106	96	87
Rb	28	31	31		18	27	21
Sr	117	152	112		121	108	127
Mo	32	43	48		116	62	43
Cs	17	16	18		10	15	17
Ba	4	8	6		5	6	4
La	7,689	9,253	8,198	5,986	9,427	8,217	7,969	7,973	7,282
Ce	10,019	11,283	10,621	9,050	11,880	10,559	10,032	10,732	10,083
Pr	802	928	849	581	927	846	783	667	786
Nd	2,362	2,734	2,577	2,092	2,716	2,495	2,402	2,298	2,306
Sm	527	574	541		543	557	532
Eu	32	41	39		38	36	39
Gd	632	714	642	902	651	669	662	1,006	842
Tb	140	158	151		144	147	146
Dy	1,009	1,088	1,079		1,029	1,032	1,094
Ho	232	247	262		247	236	254
Er	692	728	768	883	750	731	769	840	936
Tm	109	119	122		115	114	123
Yb	764	776	837	843	815	765	822	957	1,019
Lu	100	098	112		107	102	108
W	377	402	409		882	634	216
Pb	1,117	1,425	1,267	882	1,345	1,229	1,321	1,550	1,402
Th	7	6	9		7	8	7
U	25	28	31		25	29	33
Analysis	10	11	12	13	14	15		Låv	Wöh
wt%^b					14	15		Låv	Wöh
SiO₂	47.64	46.84	48.07	48.28	47.46	46.27		29.39	28.91
Al₂O₃	0.07	<0.01	0.21	0.02	0.07	0.02		<0.01	<0.01
TiO₂	0.24	0.18	0.24	0.22	0.30	0.17		3.61	1.37
ZrO₂	11.40	11.81	11.11	12.11	12.02	12.01		27.08	15.23
HfO₂	0.23	0.23	0.12	0.14	0.14	0.13		0.27	0.34
Nb₂O₅	2.41	2.57	1.77	1.55	1.28	2.56		3.38	13.55
Ta₂O₅	0.22	0.29	0.05	0.07	0.08	0.06		0.09	0.47
Y₂O₃	0.79	0.84	0.47	0.39	0.37	0.39		0.36	0.32
REE₂O₃	2.75	2.90	2.47	2.00	1.99	2.86		0.145	0.811
FeO	3.20	3.19	4.08	4.83	4.76	4.27		3.77	0.42
MnO	4.09	4.25	3.21	3.04	3.01	3.65		4.03	1.59
CaO	11.82	12.15	10.76	11.42	11.77	11.86		13.02	25.79
Na₂O	11.52	11.52	11.68	11.24	11.48	10.92		12.04	8.05
K₂O	0.45	0.25	0.43	0.37	0.50	0.24		<0.01	<0.01
Cl	1.70	1.76	1.75	1.85	1.81	1.80		<0.01	<0.01
F	0.14	0.13	0.15	0.29	0.20	0.18		4.54	3.72
H₂Oc	1.82	1.78	1.79	1.64	1.69	1.68
-O=F,Cl	0.44	0.45	0.46	0.54	0.49	0.48		1.91	1.57
Total^d	100.39	100.45	98.87	99.92	99.30	99.58		99.81	99.02
Structural formulae (a.p.f.u.)^e					99.30	99.58		99.81	99.02
Si	25.306	25.135	25.568	25.401	25.399	25.090
Zr	2.953	3.090	2.881	3.107	3.136	3.175
Ti	0.095	0.073	0.097	0.086	0.120	0.069
Nb	0.579	0.624	0.425	0.369	0.311	0.627
Fe	1.421	1.432	1.814	2.124	2.129	1.937
Mn	1.841	1.930	1.445	1.356	1.366	1.676
REE	0.544	0.574	0.477	0.381	0.394	0.562
Ca	6.728	6.984	6.129	6.434	6.746	6.891
Y	0.222	0.238	0.134	0.108	0.106	0.112
Na	11.858	11.979	12.041	11.463	11.914	11.475
K	0.307	0.169	0.292	0.250	0.339	0.168
F	0.240	0.217	0.252	0.476	0.337	0.309
Cl	1.531	1.599	1.581	1.647	1.640	1.657
H	3.228	3.184	3.167	2.877	3.023	3.034
Trace elements (ppm)					3.023	3.034
Mg	127	60	1,547	1,179	3,582	1,301		3,819	083
Sc			29	27		25		63	35
Zn			831	732		852		329	27
Ga			76	61		86		<01	23
Rb			121	73		56		<01	<01
Sr			2,161	2,315		2,699		75	12
Mo			41	41		106		18	24
Cs			39	31		27
Ba			3	1		4		<1	<1
La	7,418	7,930	7,967	5,868	5,679	8,993		54	1,018
Ce	10,519	10,912	8,639	7,120	7,539	10,257		120	2,515
Pr	743	803	675	586	675	817		14	304
Nd	2,435	2,452	1,870	1,750	1,938	2,325		56	1,179
Sm			362	343		413		24	304
Eu			27	24		28		3	20
Gd	685	937	402	355	564	427		65	288
Tb			74	64		73		26	64
Dy			465	403		445		251	449
Ho			94	78		94		66	98
Er	944	1,032	234	215	350	238		216	299
Tm			33	34		36		38	47
Yb	755	711	237	226	307	241		294	337
Lu			26	28		27		36	45
W			232	353		576		7	127
Pb	1,439	1,374	1,537	1,644	1,476	2,011		19	52
Th			58	112		130		33	9
U			12	8		7		21	69

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[TFN1] ^aEGM-bearing nepheline microsyenite dyke (centre); ^bEGM-free nepheline microsyenite dyke; ^cMelatheralite (wall rock of the EGM-bearing nepheline microsyenite dyke); ^dPeralkaline index: molar (Na₂O+K₂O)/Al₂O₃.